We have a 2013 390BH that we got in January. We live in Texas and have had a couple of days approaching 90F with high humidity so far. When we first got the coach, I was quite concerned about this as well - particularly with the heat coming in through the windshield - even with the interior sunshade in place. One step we took was to install an exterior shade on the windshield and front driver's side window. My wife made them essentially as clones of the ones from MagneShade for significantly less money - maybe $250 for both. They have made a significant difference in keeping the front of the coach from overheating. With that, I feel the 2 smaller AC units will be okay. I'd love a third, but suspect it won't be a big deal. We keep the coach around 74-75F during the day and 70F for sleeping. We have found that for rapid cooling, opening the main dump vent on the unit works very well, then closing it to utilize the ducts maintains temps well. I do wish the units were a bit quieter, but such is life!
Now in the 100F, high humidity summer, we may change our opinion...