Only once have we experienced a problem with our single A/C struggling when the temps are high. However, we don't usually camp down here in the summer. We were in PA on the side of a small mountain, completely exposed with the sun beating down on our 32' Class C with 1 13.5k A/C. The A/C ran all day, never cycled off. It was in the low 90s out. The camper stayed cool, but like I said it ran all day.
Our seasonal camp site in NJ is in complete shade and even when it is in the mid to upper 90s, the A/C handles it fine. If we camped down south in the summer, we'd definitely get a second roof top unit for the back half of the camper.
-Michael