So, what do you expect from your rig AC? We have a 13k BTU AC in the front/master and a 15k BTU in the living. At present, the main 15k AC is keeping the rig at 10-12 degrees lower than the outside...
Don't forget to check the interior plenum divider. These are notorious for being poorly installed allowing cold air to re-circulate in the system.
Mine also leaks cool air into the roof cavity. Not real excited about pulling the ceiling apart to fix this. Maybe if I was in TX full time.
For reference my 26' cooled by a single 13.5 will hold in the low 80s inside when it is 105+ outside. As long as it cools at night when the sun goes down we are OK with it.