The simpler answer is a third unit, if you have an available roof vent. Also, don't waste your time taking the A/C in to be checked--it works fine, I'm sure. As other have said, there is a limit to what an RV A/C can do, particularly if it is ducted.
If I was in your shoes, here would be my thought process:
#1. Are both A/C units running on compression 100% of the time? If either unit is cycling, then you have TOO MUCH A/C capacity for the volume of your unit--or you have insufficient circulation of cold air in the cabin.
#2. I would check the ducting for leaks that are pretty much guarranteed to be there. If I really felt like fiddling I would tape the ducts up, or
#3. I would disconnect one or both A/Cs from the ductwork. Let them blow cool air directly into the cabin, and use a portable fan to circulate it.
#4. I would add the third A/C, if nothing else worked. IMO, the most important thing about an A/C is that it stays on compression 100% of the time. Don't give it the opportunity to pour wet, clammy air into the cabin. If you don't need so much cooling, turn the third A/C off and make the other two run 100%, and then turn the second off, leaving the first to run 100%. In my 18-ft TT I run a 6k (6,000) btu A/C for hours on end--and I stay away from Texas.
