Often, ice buildup can happen quickly if the door isn't totally shut or if part of the sealing gasket has been compromised.  Since you state this hasn't happened before possibly one of these are your issues.

Open the lower vent cover on the outside (behind the Dometic fridge) to access the rear of the fridge.  Along the metal propane supply tube (before the combustion area) you will find a small plastic tub with a clear plastic tube running to it.  Check to see if the tub is full or if the plastic line running to it is plugged or kinked somewhere.  If necessary, remove the plastic line and run some compressed air through it.