The ONLY answer in Texas Heat and Humidity is a 3rd AC on a 42 foot RV. There is NO WAY a 15k and a 13.5k will cool a 42 foot in Texas heat. ALL our RV's we sell over 40 foot have 3 AC units and my coworkers 41 foot DRV 5th wheel came with 3 15k AC/HP units and he has 4 slides and those units CYCLE and keep his family comfortable in 105 Texas heat. They are FULLTIMERS with 2 teenagers. Most of our 40 foot down to 36 foot have 2-15k units.
Last, a AC system does NOT have to run 100% to keep the moisture level and inside humidity down. A correctly functioning/sized AC system will cool most RV's and cycle. That may not happen during the 12 hours of the day at 100 degrees but will once the sun goes down. The BIG problem is what another poster stated---The INSTALL and how the gaskets and plenum system is sealed. ANY leakage will degrade the performance in hot weather and from MY observation, on systems that have the standard large return grill/filter in the ceiling, NONE are completely installed correctly. There is always room to seal them better. Doug
