If I understand your problem correctly I'd suggest running the pump a little slower so air can escape. A fast fill up traps air and it eventually has to go somewhere. When I had my 55s and 56s I could pack another gallon in after a minute or two if I fast filled it trapping air in the tank. Even my late model truck is that way. Packed a gallon and half in it yesterday after the handle clicked off since I had the handle set to fill up fast. ....take it slow and easy.
This is what works for me ! Say the tank is half full ,8 gallons I put in 7 gallons at normal speed and then very slowly until the pump shuts off. Do not try to top off. I also listen to hear as it starts up the fill tube.