Question concerning condensation

I wish they had a separate category for those who winter RV in places like North Dakota!! My 2nd winter here in an RV and ice intrusion is killing me!!! I know that part of the problem is lack of overall insulation. The front of the 5th wheel is the worst. Ice in every bedroom cabinet. I do keep the ceiling vent open slightly. I have to because I run a Mr Heater part of the time. Doesn't help though. I have tried keeping the bedroom closed off, but I think that makes it worse because there is a large gap on the bottom of the sliding door that cannot be covered. I tried 1/4-inch foam board covered with Reflectix and that just allowed the cold to turn the Reflectix into its own condenser. I tried larger foamboard on the inside, but lost all my cabinet space. It did help reduce the ice though. I need to get real insulation under the shell, but I'm afraid to tamper with the shell. Is there anyone who could advise me on how to get the job done, who to hire, can I do it myself by breaking down and replacing the cabinets and walls? So many questions and so little insight. I know I'm asking this 5th wheel to do what it was never designed to do, but I'm in it now. I would like a smaller bed than a queen and the cabinets are too short to hang anything but the shortest shirts. I could use a remodel, so I'm not afraid to break down the cabinets. Just afraid to break down the walls or the shell. I need advice!!