Did you try running just the fan? Did you measure the voltage at the RV breaker panel? Measure it before turning on the fan portion. Then measure it while someone turns the AC portion on.
I highly suspect the voltage will be low. Then go to the AC outlet you are plugging into and if it is a dual socket you can measure the voltage on the empty one while trying the test. If it does not have a second socket you can pull the plug out just slightly and measure.
This seems to be a simple Ohms law issue. Too much resistance somewhere.
