Going with a rooftop RV air conditioner the newer ones, at least, all fit in the same size opening which, I think, is 14x14 inches. Depending on your camping area, a 15,000 BTU unit might be the best bet. I have a 13,500 unit in my Classic 30 and, living in Florida, would like to have more capacity either in the form of a bigger unit or another unit in the bedroom.
Choices are limited. Dometic has several lines. Search the AC subforum on here to get an idea of what people like and don't like.
"You cannot reason someone out of a position they have not been reasoned into"
Al, K5TAN and Missy, N4RGO WBCCI 1322
2002 Classic 30 Slideout -S/OS #004
2013 Dodge 2500 Laramie 4x4 Megacab Cummins