Ok, here is what we had to do. We live in South Texas where it is really really hot. We have a 2011 Coachmen Concord. It came with a 13,500 BTU unit (ducted). It wouldn't cool it, so I had it swapped out at my expense with a 15,000 BTU unit (ducted). With a 100 degree day with no shade, it would still get to 88 degrees or higher in our coach. And yes, we had the AC unit checked out. So, I gave up and had a separate non-ducted unit installed on a 20 amp circuit that plugs into the coach and post at the RV park separately from the 30 amp plug for the coach. Now, we have NO trouble cooling our Class C at all. I highly recommend this!
Jim