Windows, walls and insulation question(s)...

I am itching to build something, working on the design, looking at windows and doors... I found a CL add with used RV windows for fairly cheap. My only issue is they are for a 2" wall thickness. I was planning for a thicker wall with more insulation, as I plan to camp in late fall as well as early spring and of course summer. I was planning on a stick built frame out of 2x4s. I would like it strong enough to walk on the roof. I was planning on 3/4" ply out side, about 2 to 2 1/4 thick insulation, with solid pine for the interior. Planning on this being removable type of teardrop on a larger frame( like a truck camper). I don't plan on going super offroading, but it will see fields, some ruts, and less that friendly road conditions. My deck is higher than normal
, I am looking at about 32" deck height. My tear about 120" long, x 80" wide, 54" height from deck to top of roof. With thicker walls, I can't see using the 2" RV wall thickness windows, I am not even sure what would be the best way to achieve that keeping the build strong. If anyone has suggestions for the wall build, or windows, I'd love the help. I know some use a skeletal frame, but I was looking at simple 2x4,ply outside and PMF around it. Windows I am considering are 30x20 screened exit RV windows. Is there are better option, or am I best off changing to a thinner wall? Weight is not much of a issue, trailer can handle 7,500. I am having the springs reduced to carry only half that. Suggestions please?!?!
Thank you in advance. - Vajra

