96 fever here...all new sending unit and gauge...fuel level stays on full while on the water...the very minute she goes on the trailer gauge reads a pretty accurate level ..

They sit vastly different in the water vs on the trailer.

Fountain fuel gages have always been a mystery. To start with they are cut to match the V of the hull, means the lower they go the more they are off. They sit mid hull, that means they are very effected by balance on a trailer. In very simple terms LEARN YOUR BOAT. When mine says 1/4 fuel I need to be looking at a fuel pump real close by.

Yep, sender is close to rear of tank, so when bow is up (i.e. in the water) the fuel runs to the back of the tank. Perfectly normal for a Fountain (and other makes with similar design).
Once I see 1/4 tank on the water, I'm headed straight back to the ramp.