You need to remove the rear fiberglass
(door side) seat, it has screws along the back side, two coming in under the sink area and a few along the floor. If you purchase the tank from Casita
or Little house customs it should be as simple as removing the old tank, moving the two floor supports / anchors, getting a larger metal clamp and placing it back in place. Fairly easy job. The fill and connections should all line up.