Bad Door seal if it's overly icing also it ices up more the less that is in it.
Every time you open door moist air enteres, the moisture condenses out and freezes,, On your Home Sticks and bricks type residential fridge there is a heater believe it or not which melts that ice and a tube that lets it drain down to the fridge and from there out onto an evaporation tray.
But on the RV absorption cooling unit there is no heater to do that (there is a heater but just around the door seal to prevent moisture there)
(oh and one in the ice maker).
Solution is to check the door seal. if it's good. DEFROST from time to time.