You might do better by posting your questions under the "Vintage RV" section in the community forums section of iRV2. It's special for rigs over 20 years old. You'll probably get more responses.
I had an '86 HR presidential with similar problems. The very first thing I would change is the engine driven fuel pump. Mine was wore out and often sputtered going over overpasses. A new fuel pump from Car Quest solved that problem.
It's tricky holding up the push rod to insert the new pump. I used a thick grease smeared on the push rod, then lifted it up with a screwdriver and quickly inserted the new pump. For $40 bucks or so, this might solve your problem. Could be water in the tank, or old gas. I don't know if it had an intank pump- someone else might chime in on that. I quit looking for more problems when the new fuel pump solved my problem.
I don't know about the gas gauge problem.
Brakes were a problem on mine. The front wheel rubber brake hoses had collapsed internally and created "pulling problems". Replacing the hoses solved that problem.
The interior ceiling sagging was an easy fix.
After I took it to several Auto upholstery shops and the guys just scratched their heads I quit looking for outside help.
I bought several lengths of "threaded rod" at Lowes. I think it was 1/8", or so (maybe 3/16"). I then drilled the same size hole, as the threated rod, up through the headliner side of the support "rails" (for lack of a better term) up through the roof. I also bought some ormanental washer for upholstery at Lowes. I put fender washers on the roof side of the threaded rod and the upholstery washers on the inside of the ceiling rails. I painted the inside upholstery washers and the end of the threaded rod, and small nuts, the color of the ceiling decor. I then cut the threaded rod off on the top (roof side) after the fender washers and nuts were installed and tightened to pull up the interior ceiling. I think I used five, or so, of these "long bolts" (if I can call them that), on each rail. A drop of "Locktite" on the nuts on each end of the threaded rod will ensure they won't work loose over time. Then you will need to use Dicor self leveling sealant on all the roof ends of the "long bolts". Get that at Camping World.
I had to make a special drill bit to drill such a long (deep) hole through the ceiling. I took a 1/8" drill bit and brazed on a piece (maybe 6'' or so) of round steel stock, the same size as the drill bit. Survey where the holes will go through the roof and place your holes accordingly. You can drill the initial hole through the upholstery rail with a normal length drill bit and then push and turn slowly, by hand, the long drill bit to avoid any wires or such. When you get to the roof, attach the electric/battery drill and go through the roof.
Cleannup around your skylite, scrape and remove any old caulking and then again, use the Dicor to reseal it. Removing the skylite might even be better.
Several other problems I had,
1- Steering "wandering". I had mine realigned three times. It didn't help. The front end integrity was sound. My "Sagniaw Steering Box" needed the adjustment screw (on top of the steering box) turned inward about one full turn to adjust/compensate for years of wear, or misadjustment from the factory. When I bought the rig it had 24,000 miles on it. If you do try to adjust this, do it 1/4 of a turn at a time- test drive it, until the play is gone- go no further.
2- Your MH sits on a Chevrolet P-30 Chassis. You will find the tag axle drum brakes are FORD. I think F-350. Wheel cylinders and brake shoes, will match
up with Ford parts for the "Tag Axle".
3- You will have a seperate master cylinder for the tag axle. Mine was located aft, next to the steps at mid-ship (under the floor). I had to use a turkey baster with a piece of plastic hose to refill the master cylinder.
4- I was using more oil than I thought necessary so I switched from 10w-30 to straight 30W and that solved that prpblem. The 10/30 was just too thin when it got hot.
5- I put the "Bank headers" on in place of the stock manifold and then thought I could use the MH as a "get away" vehicle. They didn't do much for gas mileage. I took off all the smog stuff as my state didn't require that for inspection on that old a vehicle anyway.
My brakes were never any good, even after a complete rebuild of the system. Adding a "brake buddy" to my "Toad" helped me stop better than anything I did to the brake system.
You have zeroed in on a major project, my friend. I wish you the best of luck,