In our 31 foot Springdale TT with no slides, the original 13,500 BTU air conditioner did just fine. Never had a problem. But we did not camp in 115 degree climates either.
Fast forward. Our 35 foot Outback with 3 slides and the original 13,500 BTU air conditioner just could not keep up (even in milder Indiana temperatures). We swapped it out for a 15,000 BTU because our TT is 30 amp and it would have been a major rework with the wiring to get it converted to 50 amp. So we went with the 15,000 and it's worked great ever since, even with the 3 slides.
I have no clue how a second air conditioner will run on your generator. I have a small generator and it ran the 13,500 just fine. BUT the 15,000 won't start with the generator now. We never boom dock, but I have the generator just in case. Actually use it more for the house when we have power outages than anything else.
This would be my advise, get a single 15,000 BTU.