OK. Questions first: If I were doing this trailer as a camper only (maybe even if I were doing it as a combo), I would use my 2x3s across the frame, then 1/8" ply, then 1x4 frame with 3/4 pink insulation (where I used the 2x4s and 1-1/2 pink), then top it with 1/4" ply. I would glue and screw everything.
I hope that helps, John. Please don't think me an expert here.
And now, the much delayed and highly anticipated update.
When we last left our intrepid campers, they were getting ready for the Hoosier Spring gathering.
This was Wednesday night:

I started Thursday morning by skinning the sides with 1/4" ply.

After that I secured the truck cap using 1/4" bolts and tee nuts. Here is our youngest, Lanee', doing the "trailer dance" with Preston.

Then I framed in the back.

And skinned it.

I left the space open for the ingress/egress door. Sorry, I didn't get pics of the door construction. It is 2x3s on the flat with 1/4 ply skin and 1-1/2" pink foam. Here it is installed

I covered it with a two part Epoxy Floor Primer I found at the Habitat ReStore for $2.50 a quart. I got 2 quarts of part A and 2 quarts of part B. This was difficult to use as it was a warm day and part A and part B were hardening almost on the brush as I was painting the trailer.

That is where I left it on Friday afternoon, back in May.

Unfortunately on Saturday Lanee' woke up having severe asthma problems.

We had to cancel out on our first camping trip in the Patrol Box.
Well, this has been a long post and it's late, so it's off to bed. I'll catch it up later this week. Pics

are in my gallery if you can't wait.