If it is mounted like a fresh water tank, then you will have a frame with the rear facing section being bolted on. Undo the bolts, and pull out the framing piece.
You should be able to then slide out a plywood sheet, which holds up the tank.
This sheet might be very very tight inside it's framework, possible swollen and rotten if teh tank has been leaking.
If you don't see this frame work, then you will have to drop the center section of the belly pan, behind the rear axle to the bumper. Access the tank this way.