We have a 35ft class A with a 15k and 13.5k btu. Both can keep it 69* with no problem when it was close to 100 outside and humid during July. We do use reflectix material in the windows and added a thermal curtain behind the front curtain. But it cools without a problem.
Now it may take it all day into the night to get to that temperature when we first set up, but it maintains it once it gets there.
And I was told by our A/C guy that the A/C should cool the air 20* cooler than the return air, not the exterior air temperature.
2004 Sea Breeze by National RV - 8341
Former Coaches:
2006 Keystone Zeppeline 291 - TT
2000 Aerolite Cub F21 - Hybrid TT
1991 Coleman Pop Up
Formerly known as: hybrid_camper