My buddy used to have a lot of trouble filling his rear fuel tank in has 87 class C Ford. I took it apart (after a frustrating 1000 mile drive in it) and found that the vent line wrapped around the fuel filler line. It would get a tiny amount of liquid gas in it, blocking the air leaving the fuel tank, and as a result the first 15 gallons would go right in, but then it would take 'forever' to get another 10 gallons, and even longer to get the last 3-4 gallons so that the fuel gauge would read 'full'.
After the repair, on the way home, we could get up to 25 gallons right in, and it would read full. One stop, we got in 33 gallons! He had reached 'E' and kept driving the last 20 miles into Barsto. We did not run out, but was able to fill it all the way.
I installed a second tank, now he can run the rear one dry and get up to 40 gallons in it! So we still had 7 gallons after it reached "E" and after driving another 20 miles into town! We where getting around 7.5MPG. So filling up every 15 gallons was a REAL Pain! The forward tank can hold 22 gallons, so now he can use all of both tanks, or at least all of the rear one, then 3/4 of the forward one, and still have about 4 gallons left to find a gas station! Then fill up with about 50 gallons! So much better than filling every 100 miles.
I hope that you get the problem straightened out!
Fred.