Finally a report!!!!
Sorry for the delay, but it's been hot and lots of other stuff going on.....
Now this applies to my 2007 28 TRW Excel, and 'maybe' to yours!
OK. First misery was to remove the poly underbelly without destroying it! The underbelly is held in place with 2x2 wood strips on both sides and front and rear.
Here is how I did it. First remove all the screws (about 8) from the bottom access panel that hangs below the belly near the rear and provides access to the dump valves.
Then remove all the 1-1/2” screws from the flat iron supports on the front (4) and side (8 per side) and the screws in rear frame vertical tin shield ( about 4). Now in the front, the belly fabric is just wrapped (not stapled) to the 2x2 so the front 2x2 can be removed with a little work.
On the sides, the underbelly is stapled to two about 7' 2x2 boards. Each board is sitting on top of 2 welded 1-1/2”x18” pieces of flat iron. 4, 1-1/2 screws secure the 2x2s to each flat iron (8 per side). The problem is that the 2x2 is captive between the steel side frame and the tank side near the rear of the tank, and also the flat iron supports and cannot be removed!!! See pic below:
My solution was to flex the
curb side rear flat iron (not both) of the flat iron supports back and forth until I could break the welds. The 2x2 was then mostly loose but still couldn't be removed, until I discovered that at the factory they had installed one screw into the 2x2 from the INSIDE, through the 2x2 and into the inner metal frame!!!! Damn! This was about 12” from the rear of the long 2x2. This required a saws-all (a cold chisel would work too) to chop the screw and then the side 2x2 finally came loose, without harming the poly or the 2x2. (Note that I decided to leave the street side 2x2 in place and was able to complete the entire repair leaving the belly attached on the street side)
The rear portion of the belly was also held in place with an adhesive, but but pulling carefully the adhesive released the fabric and now I have the belly loose on three sides. I then folded and slid the belly as far as I could to the street side. This way I did not have to cut any of the belly or the 2x2 wood, and did not disturb the dump pipe that goes through the belly.
Now with the belly aside, I saw several broken tank straps!
As I see it, there were two problems: First there were only 2 straps on each tank. And like Rockin' Tom I repaired with 4 straps on each of the two tanks and used a better gage strap.
But the main problem is poor design of the installation. The rear of the straps was held by a single screw that was screwed vertically up into the bottom of the rear 2x2.
This means that the strap was bent and stressed by the screw head and failed as above!
Detail of tank rear new replacement strap(s) is above: First the new strap is around the floor 2x2, up and over the top. Although hard to see in the pic, the end is folded back over itself, doubling it up and is held by two screws, one on the vertical and one on the top. I expect no stresses from the screw heads this way! This is almost a no cost improvement that Peterson should be using for tank strapping. (And maybe they are or have changed their methods)
Now the installation and tensioning of the straps at the front end of the (RV) tank:
These strap ends are installed the same was done at the factory, with the exception of the doubling of the strap ends as show above. This in effect serves as a washer under the screw heads to spread out stress, although no cracks were found in this area in the OEM straps. This doubling over of the ends is again almost a no cost improvement for the factory.
Tensioning is as follows: Tighten the strap as much as possible by hand or pliers and install screw #1 and tighten. The strap is now forming a straight line from screw #1 to the bottom of the tank on the right, and is about an inch away from the anchoring wood about where you see screw #2 in the pic. Now start screw #2 and as you tighten it will draw the strap tight. If you can't 'play a tune' on the strap, you may have to re-do the tightening. I did re-adjust several, as other straps added their pull.
And as Rockin' did, I have 4 straps per tank instead of the factory 2.
Ahhh, but now I still had a crack in my black tank that started my investigation:
This is the inner side of the rear of my black tank. The crack was found at the top of the tank where it transitions to the upper flanges.
Analysis: I suspect most of the problem and the cracking occurred, after the outer support strap broke, allowing the tank to flex, but I can't be sure. The metal you see on the left is factory made channel out of medium gage metal. It supports the inner flanges of the “side by side” black and gray tanks, from the front, to the rear of the tanks. FYI, the outer flanges of both tanks is supported by a factory made medium gage angle that is screwed into the side frame of the RV. Although this home-made angle starts at the front of each tank, it ends maybe 18” from the rear of the tanks, leaving the outside rear flange unsupported except for the straps. I don't know why this angle is not extended to the full length of the tanks. There may or may not be a good reason, but it makes the strapping critical, IMHO.
Anyway, rather than go to all the trouble and expense of a tank replacement, I opted for an in-place repair. I purchased a repair kit from
http://plasti-mend.com/default.aspx:
They say:
Plasti-Mend products are a specially formulated mix of plastic resins and solvents designed for plastic repair by topical application. It works by chemically melting an application of plastic resins into the existing plastic, creating a new layer of plastic bonded into the existing plastic. This, with a mesh embedded at the time of application, makes the plastic repair many times stronger than the original item.
I bought the small $18.76 kit and plus shipping cost $25.45. I cleaned and sanded the area to remove dirt. Tried to rough gouge the crack with a piece of hack saw blade. (a dremel tool would have been better but I didn't have one). Used their special cleaner (it dissolved the cheap rubber gloves I was using!). I then applied one coat of the Plasti-Mend resin, added a piece of supplied mesh and covered with resin, waited 2 hours, and added a second coat, and 2 more coats at 2 hour intervals and I was out of chemical. I am well pleased with the repair-in-place. But only time will tell!!! Be careful of the dangerous organic chemicals. You need to ventilate and where a mask!
Now to close up the poly underbelly. The 'Original' side flat irons look like this:
Since I destroyed one of them (curbside rear) during the underbelly removal, I purchased a piece of 1/8”x3”x18” flat iron and drilled 8 holes in it. 4 to allow bolting to the underside of the main frame box channel instead of welding, and 4 to allow re-screwing to the 7' 2x2 wooden support piece:
In this way, I was able to first get the 2x2 with the poly back into place along the curb side tank edge and then put the new 3x18 flat in to hold everything together, as you see above.
Here were my steps: (BTW, I discarded all of the insulating fiberglass that was in the underbelly – if I'm in weather that cold... it's MY FAULT!)
Remember that I left the street side 7' 2x2 in place and reinstalled the 8 screws that retained the 2x2 in place.
Next I laid the front 2x2 into the fabric (which was not stapled but the belly poly fabric was just sort of rolled in the 2x2). Working by myself, I put a couple of temporary screws into the front fabric & 2x2. Eventually I was able to reinstall all 4 front screws in the supporting flat iron.
Next I temporarily inserted the rear 2x2 into the fabric, unsecured, and let it sit there for a while.
Next I stretched the curb side 2x2 and still attached fabric to the side. I inserted the 7' curb side 2x2 into general place and with my better half helping me by squeezing the 2x2 to the outside frame with a large set of water pump pliers I was able to get a temporary screw into the front curb side flat iron. Next I installed the rear curb side new bolted flat iron by bolting to the frame. Now, again with the large pliers, I was able to get a screw started in the new rear flat iron. By moving front to back and again, was able to get all 8 screws installed on the curb side.
Lastly with spouse's help I reinstalled the 4 or 5 screws from the rear metal plate into the rear 2x2 while stretching the fabric as tight as possible.
Now it was back together and I use a spray adhesive to reach up behind the dump valve access panel and spray the back wall. I then reached inside and pushed the rear portion of the poly belly material against the rear metal to complete the job.
Finally, I reinstalled the bottom plate on the dump valve access frame.
Not mentioned was loosening the curb side propane line and re-securing it when done.
Finally, the original installation had one factory installed strap metal under the belly poly, from one side, to reduce wind flopping. I bought 2 pieces of 1x1 aluminum angle to serve the same function and you will see that in some of the above pictures.
I'm sure I've miss a couple of things or misstated some. Yell at me if I can clarify!
Tom
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I mean after your done working on it!!!!
It's so much that you can stand on the side of the rig a few feet away and see the bottom of the tank hanging below the frame!! 
rockin' over analyzing again.
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