Whenever you RV or camp in the South or the Southwest, the conversation should NOT be about a second 15,000 BTU air conditioner. The conversation should always be about a THIRD air conditioner.
Not only that, but each should be wired separately and each with a separate dedicated thermostat. AND, the Quiet Diesel generator should be big enough to run all of them, and then some.
IMO if you do this long enough, you'll understand the reasons.
Of course, it all depends on your individual comfort level. My wife and I insist on being comfortable, and also want our kids to be comfortable (2 small dogs, occasionally three).
I do NOT believe that 15 degrees below the outside temperature is the best that you can do. I believe that you can add air conditioning capacity PLUS insulation, and make things quite comfortable.