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...
Awnings over every window and door. A rack full of solar panels on the roof--say 3 inches above the air conditioner shrouds. Double pane windows and beef up the (almost no) insulation with pour foam.
It is not that the existing system is too small--it is that the heat entering the RV is overwhelming what is there.