Hi, here is a link to the Dometic heat pump manual. Check out the section on defrost.
It is possible there is an issue with the timer sequence (usually a circuit board) that controls the defrost cycle or reversing valve (the refrigerant valve the changes the flow from heat to defrost, which is actually the a/c cycle by the way). Also, low refrigerant charge or defective outdoor fan/fan relay could be the culprit.
I have worked on tons of heat pumps but Iíve never pulled one of these apart yet. I have the same unit on my rig so Iím sure at some point Iíll get a chance.
There is also the possibility that because of continual excessive moisture and cooler temps the defrost cycle simply canít keep up. Even though the air temp is in the 40s, the coil temp is way below freezing so with all the moisture, you make ice like crazy. It should be designed to handle it but like most RV stuff, who knows what design conditions they used to design these things.
In any case, with this much ice on the outdoor coil you are getting very little heat out of this unit and continuing to run it in this condition can cause damage to components like the compressor and outdoor fan motor.
You should change over to gas heat until you can troubleshoot the unit.
Best of luck,