Myredracer,
Thanks for the advice. Insulation will probably be rigid foam that is cut to fit each bay. This way I can bond the foam to the ply sheathing. Spray in foam is superior but I won't be able to bond it to one side of the sheathing very well (if at all). Before fitting each piece of insulation in place I plan to shoot a little expanding foam into the corners to ensure it's as airtight as possible. I do plan to use a little pressure treated lumber in the framing, but only for the top and bottom plates, around doors and windows and also the corner framing members - basically those areas that would be the most likely points of water intrusion.
We don't do any significant amount of cold weather camping, so I'm not that concerned about the thermal efficiency of the windows. My wife and I both like the jalousie windows that we currently have and being able to have them open even in the rain. Also, the cost of replacing all of the windows would be significant (those buggers are expensive!).
An arched ceiling looks great, but it would add a significant amount of work...not only with the roof itself but also the interior partitions and cabinetry and the roof vents as well. A simple flat roof is just fine for me.
The trailer frame is pretty stout (I think the main beams are at least 6" deep, maybe more) and I don't plan on adding any significant amount of weight, so I don't think there will be any issue there. I will certainly give the frame a good going over once it has been stripped down. Depending on the condition I may sandblast and repaint.
