Outside temps in the 20-30F range is not cold for an rv furnace to keep up with. My furnace keeps me at 70F in -25F temps. However, I do cover the doorway with a H.D. insulated drape. RV doors,can be a leaky cold spot.
When I had the same issue, I moved the lever in the center of my thermostat . It adjusted the cycle time of the furnace allowing it to run longer, giving me more heat. That was my cure and did that in 3 rv's. Simply, pop off the wall thermostats outer plastic cover, they snap in place.
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Also, if you have a bug screen cover on the outside vent/exhaust...try removing it to allow optimum air flow.
Make sure the trailers interior return vent is not blocked by something, like a blanket or a box etc.