My experience is that RV thermostats aren't very precise. Set it for 74 degrees and it will keep it within +/- 3 or 4 degrees. That's up to an 8 degree swing in the temperature that it calls for. Then you factor in balance load on the ducts (there isn't any) and cooling demands. Depending where the sun is, where the insulation is, where the slides are, where the windows are, etc you're going to get unbalanced temperature control.
We find that one night we can set the AC at 72 and be freezing and need blankets, and the next night at 72 we're both so warm that I eventually get up and adjust the thermostat. Same campground, same spot, same people... wild swing in temps.
We keep our fan turned to ON instead of AUTO to keep circulating air. That helps. Perhaps a good quality circulating fan to help move air around will help balance.