I have a FR Lexington 255RDS - Ford E450 Chassis. I too have this fill-up problem. At first I thought either my gauge or the level indicator in the gas tank was defective, but the next time I filled up the gauge went to "full". At some gas stations, I can fill to the brim no problem. At others, I can't get above 3/4 tank. I have noticed two things: It helps to have the MH perfectly level, and that slower filling pumps also allow me a greater likelihood of a full tank - but it does seem pretty random.
Because I can always get at least 3/4 tank, I'm not going to worry about it, although it did make a mess of my mileage calculations during a recent trip from Atlanta to Jackson Hole/Yellowstone/Rushmore and back. (About 8.5 mpg dolly-towing a Jeep Patriot).
A second consideration is this - many gas stations have a "cut-off" price when you use a debit (and maybe a credit?) card. During my trip, I had one cut off at $50, another at $75, and one at $90. With a 55 gallon tank, that was inconvenient to say the least (although you can restart the pump by inserting the card again).
Anyway, if I were in your shoes, I wouldn't consider any modifications unless the situation gets worse (e.g., can't fill above 1/2 tank).