We have a similar sized (38') coach with two of the same units on the roof and recently stayed on the Texas coast and didn't really have as much trouble cooling as we assumed we might considering the heat and humidity down there. Even here in DFW area it stays reasonably cool during the 100+ days.
The units never really shut off the whole trip but it wasn't above 78 degrees inside at any time. I would certainly have yours checked before anything else and make sure they're doing all they can.
I have always wondered why some form of mini split wouldn't be ideal in an RV? It seems like anything would be more efficient than those rooftop units, easily quieter as well.
I had a friend build out a school bus to take to races and used something
like one of these to cool the sleeping area. It runs directly off of 12 volts and according to him it was great.
Not the best to look at though.