Auxiliary fuel tank for 2014 Ford E350

[Onstar20]

Concerned about exceeding load ratings when adding a aux fuel tank. Anyone experienced/knowledgeable with having a auxiliary fuel tank installed on a 2014 Ford E350 Super Duty 5.4l or similar TV. I am thinking relocating the spare to a tire mount on the rear door and installing the tank where the spare is under/rear just forward of the hitch receiver. Not sure of the size fuel capacity yet hopefully ~30-40 gal. (~225-300 lbs plus tank/accessories weight ?).

I'm looking for in sights and solutions for impacts to tow capacities, handling and suspension issues if any, installation complexities integrating with fuel lines/gauge electrical, switches ETC. while maintaining emissions compliance and locating an installer local to Hemet Ca.

I am working with the following load ratings for my rig;
E350 Van 373 gears, Per manual; GCWR 13000 lbs
- Per manual; GVWR 8800 lbs.
- Per manual; Base Curb Weight (lbs.) 6028 lbs
- Est Gross Vehicle Weight loaded during towing; 6528 lbs, includes passengers/cargo currently, + ~300 lbs extra for aux tank/fuel ~6828 lbs
- Per manual; Maximum Loaded Trailer Weight (Lbs.) 6700 (note: if TT was loaded to this weight it would be over GCWR by 528 lbs)

Wilderness Travel Trailer
- Per manual; GVWR 6900 lbs
- Per manual: Dry Weight 5350 ibs
- Per manual: Hitch weight 475 ibs
- Est Gross Vehicle Weight loaded during towing; 5850 lbs, includes weighed as loaded supplies/cloths, min levels in holding tanks.

Summary of estimated rating factors;
-E350, Est GVW w/new tank; 6828 lbs
-TT, Est GVW ; 5850 lbs
-Hitch weight 475 ibs
Total; above combined weight 13,153 lbs - Per manual; GCWR 13000 lbs = 153 lbs over rated GCWR

I have not gone to a CAT scale to get actual weights but will and adjust what I carry accordingly.

Question;
Is there reserve capacity built-in in the GCWR of 13000 lbs.? A little confused here that although for the E350 it is ~2000 lbs under the 8800 lb GVWR and for the TT it is ~1100 lbs under its 6900 lb GVWR still this rig is considered over the GCWR by 153 lbs.

Any recommendations, suggestions and opinions are welcome.
Thank you.

[Skip426]

From the weights quoted , trailer hitch weight of less that 10% of trailer weight seems low.
You will be shifting weight further to the rear with the move of the spare tire and adding all the weight of the fuel and tank behind the rear axle, both of these , will pick up the front end and could easily result in a handling problem; as well as possibly overloading the rear. ( RAWR #'s and tires)
If your avatar picture is any indication , you are already light on the front axle.

You need to load the van & trailer up for travel and get to the scales.
JMHO: I'm thinking you will find you are already way more than the 153 lbs. over GCWR. Having towed over the GCWR of a previous tow vehicle and experienced , overheating and handling issues, at 1,000 lbs. over limit, but under on every axle, I can tell you , if there is any cushion in the GCWR #'s, it isn't much.

[Onstar20]

Originally Posted by Skip426

From the weights quoted , trailer hitch weight of less that 10% of trailer weight seems low.
You will be shifting weight further to the rear with the move of the spare tire and adding all the weight of the fuel and tank behind the rear axle, both of these , will pick up the front end and could easily result in a handling problem; as well as possibly overloading the rear. ( RAWR #'s and tires)
If your avatar picture is any indication , you are already light on the front axle.

You need to load the van & trailer up for travel and get to the scales.
JMHO: I'm thinking you will find you are already way more than the 153 lbs. over GCWR. Having towed over the GCWR of a previous tow vehicle and experienced , overheating and handling issues, at 1,000 lbs. over limit, but under on every axle, I can tell you , if there is any cushion in the GCWR #'s, it isn't much.

---------------------------------------
Thank you Skip for your insight and the points you made which caused me to reevaluate my calculated numbers. I hope all of this information is beneficial to others as well.
-I know I need to get my rig CAT scaled to settle actual/load distribution potential concerns.
-Regarding the hitch weight; I quoted the "per manual" weight of 475 lbs. You got me thinking that in reality using my calculated/bathroom scale methodology, I have actually distributed ~200 lbs (generators/carrier, general cargo to the TT tongue, so I am about 675 lbs (~12%).
-Regarding RAWR and the spare tire weight further back on the rear door carrier and the added fuel weight behind the axle; per Ford manual Spring/GAWR Rear 6084 lbs/tires range E @ 3194 lbs per, I think I am good based on the curb weight per manual of 2413 lbs, add 500 lb cargo load bringing me to ~2913 lbs rear actual well under the aforementioned ratings.
-I appreciate my avatar picture may be misleading. The Quadvan has an unloaded rake appearance toward the rear as the wheel well cut out measurement is 40" rear and 42" front. When WD hitched to TT the wheel well measurements are within a 1/2" variable from unloaded. Don't think it is light with the 4x4 system upfront.
-Skip, based on the other points you made, caused me to reevaluate my weight estimates again and I found that I should not have added the 475 lbs manual hitch weight to the GCWR AND I forgot to subtract 750 lbs when having removed the (10 passenger) 3 bench seats which now brings my calculated GCWR to 11928 lbs, 1072 lbs under the 13000 lbs manual rating.

I have had several other types of rigs over 40+ years RVing, so far I have 3 trips with this rig, ~3000 miles, (one Yuma trip in 100 degrees), and no concerns/issues with overheating/load distribution/Sway control with the Husky Cenerline. I think I solved my TT tire issues and us a TPMS. I can say that this E350 Super Duty gets very thirsty when towing, hence the need for additional fuel range capacity.

FYI, I have more info on this rig at: https://www.youtube.com/channel/UCPL...MhcvWVdq13JA0Q
Thanks again, Frank