am I expecting too much?

herefishy wrote:
Our 5er will only cool to about 9 degrees less than outside - is that all the rest of you get?? Or do I need another air conditioner?

Simple things first: make sure the ducts are properly connected to the A/C and all of the A/C's output is being properly distributed.

Next realize that a RV with minimal insulation parked in direct sunlight with ambient temperatures 90 degrees plus puts a heat load that any 13,500 BTU or even 15,000 BTU A/C will not overcome. You will need 3 maybe even 4 A/C's to overcome that heat load.

Your one A/C is doing a pretty good job under those circumstances reducing the temps by 9 degrees.