I think we're getting into hot water

The AC and DC fuse/distribution box will be inboard from the water heater. I decided to get the heater mounted before all the cabling gets in the way. Once again, we have a case of needing to do "A" before "B", "B" before "C", and "C" before "A".

The heater is an Atwood propane fired on-demand unit. It needs to be raised above the floor to avoid cutting the plastic outer body panels, and to more or less vertically balance the curvy part of the outer wall.

So, it will sit on a riser box just aft of the propane valves. Here I've marked where we need to slice Annie open. The riser is being used to set the height.

Reasons for choosing this location are that there is a minimum amount of inner wall to cut, and that the inner and outer wall are connected on both sides of the cut pretty close to the hole. That will help keep things rigid.

 The inner hole is cut.

Measured and marked for the outer hole, and drilled corner pilot holes.

I cut the hole from the outside. Covered the hole with a bit of plastic sheet because we were expecting rain.

Dry fitting the heater, while saving the hole cover for a rainy day. Not sure what the next step would have been if I'd screwed up the cut...

Installing the riser that the heater sits on. The riser is screwed to the floor and VHB'd to the outer wall.

Because of Ford's stupid form before function curves of the sidewall,  the vertical center of the heater hits the wall well before the heater top and bottom. In other words, you can't mate the two flush. So, I'm adding aluminum spacers and LOTS of caulking tape to convert the warped sidewall to a more-or-less flat mating surface.
Adding spacers:

Made sure the heater is seated properly. I'll be tying it down with some plumber strap as the last step.

Then the caulk tape, non-sag lap sealant, and mounting screws.

Removed a bunch of the excess lap sealant and painted it silver. That sealant stuff is a real pain to work with and finish smoothly. I pretty much failed at the task (as the aesthetics committee made very clear), and will revisit it in some of my copious free time. But for now, it sure won't leak!