It also depends on how many and how big your windows are. Thermo pane will help. Our 5th has reg windows and one large rear. Last year we were camped in 104* weather and unfortunately the rear window was facing south. It was about 99* inside with the 13,500 BTU air running. We also have average insulation. There was so much water running off the rear gutter from the air conditioner it looked like someone had a garden hose up there.
Also how was the air conditioner setup? I find that if I just depend on the unit it's self and not the vents that are scattered throughout it's more efficient. I open the slider vents on the unit and that concentrates the cool air where the unit is. Most of the time when you close the vents on the unit and try and use the other vents that are scattered around the trailer the cool air volume drops. We also close off the upper part of the 5er so the AC only cools the lower part.
For the most part RV AC's aren't the most efficient until you get are in one of the better insulated units.
