i bet there is a diverter on the main unit inside the RV. mine has a slide that i can direct air into the main cabin AND the returns...
I open it a little and then divert the rest to the bedrooms... helps a ton. AND the key to AC functioning properly is to have air moving in the room. I always have 3 small oscillating fans. in each room and the main cabin. This keeps the thermostat reading a true air temp rather than the duct blowing near the T-stat and shutting off.
I was in 120 degree heat... ya i know.. HOT and with the fans running, I kept the trailer to 85. not bad... seeing i have a 13.5k AC