...forget built in insulation[styrofoam] unless you don't mind the expense of running heat and a/c....I have spray foam in mine and also a sheet foam insulation....a 15oo watt heater and 13.5 a/c is good enough for winter and summer in my 37' trailer....it will pay for itself if you are using it a lot the first year-but won't pay if it is a vacation or weekender unite for 3 years of occasional use....maybe longer if the weather is mild....spray foam eliminates the leaking issue altogether as the foam seeps or is sprayed into the cracks and expands-stopping any air leaks or water leaks....may be repaired inside or out with a spray can or you can bring it back to us and we will respray it for you-cuz if you wreck it you are not going to be is a good mood any way so the respray is on us.....I do it for you guys as a favor.....anyway there is nothing better than spray foam in a RV and of course the manufactures wont spend the money....so you have to to keep warm or cool as the case may be ;>
....it also eliminates the moisture issue and makes it easily correctable....geofkaye