I don't know why RV tank guages & boat fuel guages are so unreliable. For the 5er I have gotten to know consumption fairly accurately by day. For the boat fuel is calculated by the engine hour clock. There is always a way around the problem when the fix is too much effort.
Boat: 32' 1996 Albin 32+2, single Cummins 315hp
40+ night per year overnighter
2007 Alpenlite 34RLR
2006 Chevy 3500 LT, CC,LB 6.6L Diesel
Ham Radio: VP9KL, IRLP node 7995