Water tank access

[Zigzag03]

My "new" 2001 Winnebago Chieftain has weathered the miles and the years pretty well, but for one (so far anyway) spot. The compartment that houses the water heater is just aft of the driverside slide, in front of the rear wheel. It is rusting out from the bottom. While it's not imminent, it will need to come out and be fixed sooner than later. There's carpet above it on the floor, or there's the external bodywork, but I'm not sure how it was intended to be accessed. Anyone have any experience with this problem? Thanks!
Sorry, can't seem to master getting the first pic properly oriented...

[Zigzag03]

Obviously I've titled this wrong, it's the water heater, not the water tank, I'm looking to gain access to. Don't see a way to edit it now...