I bought a truck camper last winter. I started testing it in winter conditions as cold as 9 deg F and moved into it full time starting on Mar 10 with water on Mar 15th. I installed a digital thermometer with one internal and four external sensors to ensure my tanks did not freeze.
It a a Bigfoot 3000 series rated to -20 F and well insulated with thermal windows. I had insulated covers for the skylights windows and door lights but only used them a few nights as they made it dark inside. For me and two dogs it worked well. I used propane for heat and would burn through a tank in 1-3 days. I would get a week out of two tanks as it warmed up a bit. The furnace worked great. I have an emergency exit over the bed and there was a gap on one edge which when I tried sealing it resulted in some condensation there. I ripped off a portion of the foam seal to let in more air or let it out and my condensation issue went away. I never had a condensation issue. I would sometime turn on the stove fan when cooking or boiling water. Before I filled the water tanks I found the furnace took the temps up from 9F to comfortable in 30 minutes and toasty in a hour. Some of the surfaces were still cool.
While I debated adding some 1/2" or 3/4" foam board insulation under the bed and in the Pajama lockers I worried this might cause condensation under neath so I deferred trying that. It might help for your situation. Also putting a 2" piece under your camper would help. My cabover area was coldest at the headboard the pillow blocked heat from reaching there pull the pillows forward to allow air to circulate there. I also installed a marine fan on one side up high to push warm air that direction. It has a timer so it shuts off after 2-4-6 hours. It work well in warm weather also.
I do not have electric heat but I am considering it. One issue is circulating the heat to the basement. If you want to use electric heat it is possible with circulation fans or possibly heating elements in the basement areas. I've thought about how to keep it warm on propane for longer periods. I have a Superhitch and extension and a small trailer that could hold larger tanks. I could use the propane line to the generator compartment to hook up external tanks and a two stage regulator.
Many people complain about propane and electrical use with a furnace. I have no problem ls with electrical use. I have 630 watts of solar on my roof MPPT and two (2) 200 AH MasterVolt slim line batteries in my battery compartment. 236 lbs. this also allows me to run an electric blanket with two sides. Each side uses 1 Amp per hour per number scale. I warm the bed on high for 15 minutes, dial it back to. 5 for a while watching TV, and then 1 overnight. I can set my furnace thermostat lower and since I typically heat only one side and the dogs cuddle up on my feet I don't see much difference with energy use compared to the furnace only. I love my electric blanket. It you want luxurious comfort I recommend a top notch solar system and E-blanket. Perhaps a heated mattress pad would work better. I was concerned about opening two drawers.
Insulation is very important. You really need to go crazy gluing in extra insulation, taping off gaps and fitting small pieces on access doors. I plan to add an inside fill for water and carry in a few gallons at a time. This way you can add insulation to the exterior fills. Foam pillows with Reflectix panels will help a lot for ceiling vents. Personally I like the light inside my camper. I would block these only when it is expected to be extremely cold at night or when I was away. The bubble wrap idea is a good one because light is important. I also switched every bulb to LED. I can leave more lights on and the will make it nicer too. I am considering a curtain by the door to limit heat loss. My furnace seem to do a great job of heating thing up do that may not be necessary. The bathroom does not have a vent. I am considering adding one with a at there.
My generator compartment is empty. I debated hooking up two more propane tanks there with vents added.
The bottom line is you can do it. I recommend Bigfoot 3000 series with no slide-outs, lots of solar panels, 400 AH of batteries, a water heater that runs on DSI gas and electric, electric heaters, extra insulation wherever you can. Redundant heat sources, a good battery monitor, temperature monitors and spare parts. Good luck.