When you are ready to canvas the interior hit me up I can give you some recommendations to make it easier.
For a switch plate I use these
https://www.lowes.com/pd/CARLON-1-Gang- ... 1000976020 . Just place it on the wall where you want it, trace around it with a sharpe, hot wire the foam out deep enough for the box, glue in with DAP Latex Caulk. They work great!
For gluing in your hard points I would recommend using DAP Latex Caulk or Gorilla Glue Construction Adhesive. It does not expand like the Great Stuff Does. I use to use Great Stuff a lot, but now I hardly use it at all because of the expansion issue. If I have some big voids and I want to use glue I will use it then. I like to use a bit around door and windows once they are installed. A small squirt will help keep the window/door in place going down bouncy roads.
I usually install the exterior canvas, now, once the body is assembled. I found it much easier to flip the body so I can install the canvas on a flat surface, rather than trying to apply the canvas to a vertical wall. The exception is the end walls though since it is hard to flip the body that way and still be able to work on them. It is also easier to glue the bottom flap without glue running down your hand.
Bottom floor coating is last just before applying the drip edge.
The hatch on my #1 is quite large and it works quite well. It came in at only 25lbs. so it is easy to lift without lift struts. I used flashing on the ends for attachment points and support for the latches and hinge. No other support was needed.
Good luck, George.