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...
Camping in FL in July with the rig in my sig line...single ducted AC unit will easily hold 75 degrees. I don't believe our rig or Dometic AC unit is anything special so I'd have to wonder if the OP has something not working correctly. However, if I leave the bathroom fan running by accident, the unit will never cool well enough. Also, if the entry door keeps opening, closing, being left open, etc., that doesn't help either.