Lifting the box of a class C off the frame is usually, for all practical purposes, impossible. Often they are welded together and not really separate units. Even if they are not welded together, it's been built up on the chassis and not designed to be separated.
It may be possible to patch the tank, depending on where the leak is. It may also be possible to cut a hole in the floor and get access from the inside of the RV. It may be infeasible to repair or replace it with an identical tank, but perhaps possible to have a somewhat smaller tank constructed that could fit into the space.