MY experience of high temps, with humidity, is at the Colorado River in late August/early Sept, and the temps are usually between 105 and 120, with about 60-70% humidity. My old 26 ft toy hauler had one 13.5 air conditioner, and it was adequate if we placed form-fitted Reflectix covers on all the windows and skylight. It worked well, even if it took a couple of hours to get it cold (70-73 or so). Just left it on day and night.
Now, with the 34 ft Class C, we have two a/c units, and I'm glad we do. Cools faster, and stays cool without running the compressors 24/7. When we bought the motorhome, we made Reflectix covers for the windows. I had it out at the river last Sept 1 at 115 in the afternoon, and it was very cool inside. Remained easily at 73 all afternoon, and did not run as much at night. The compressors cycled far less often as the old toy hauler, and the whole unit seemed much happier with the arrangement. I also added Reflectix to the air boxes on both AC units before we did the trip, and they work like a champ. There's a thread on that here on this forum somewhere.
