I'm not planning on doing the whole floor- it's in really good shape other than the rear corners. So all I'm doing is replacing the one panel that was damaged.
(at the same time, due to the stepped nature of the design, I'm actually looking to gain some head room space in some places).
In the end, it appears that what I need to do is find a suitable spacer to make sure the floor is level, and then worry about the insulation later. Part of the new floor is going to be covered underneath by a new gray water tank.