MNtraveler questions....
Sorry for the long delay... it seems I've been spending time on other boards lately..
Regarding your questions..
I have camped in -20 for 3 to 4 days during the John Beargrease race and I have had issues with condensation... not so much from the hard roof. Mostly I get frost buildup on the canvas and some on the aluminum frame on the roof which is exposed on the inside. It doesn't seem to be an issue until I want to fold up the canvas. To help reduce some of the frosting, I hang thermal blankets on the inside of the side walls and top canvas. This adds a extra layer of insulation but does get frozen to the outer canvas after a couple days. Just before I take down the camper top I crank the furnace and heater up full blast to thaw the canvas out so I don't crack it folding it up. This photo shows the way I hang the moving blankets on each side...
I use a kerosene heater so I have the windows cracked on both ends of the camper to regulate the heat (my heater only has one setting, FULL BLAST!) I don't use the forced air furnace unless I can plug into 110vac power somewhere,as it sucks too much LP and electric power. The kerosene heater runs without power for up to 8 hours on a single gallon of fuel. I have a combo CO/smoke detector to keep me alive... You can see where I put the kerosene heater for winter camping in this photo..
The forced air furnace is 30,000 btu I think... more than big enough to heat that small space. It's just to bad it takes 12v power to run it... It can keep the camper at 70 running 15 minutes very hour when its below zero outside... as long as no one opens the door! The minute the door opens all the heat pours out and it takes another 30 minutes to warm it up again. The 20 lb LP tank lasts a couple days at those temps.
I have not stayed in the camper with the top down yet... you would end up sleeping on the floor since neither side is big enough to sleep on unless the sides are folded out. Its doable but much too tight for me.
Typically, the Bethany is designed to be tied into the battery on the truck, and does not have it's own battery. You could put a battery under the sink cabinet, but space is pretty limited. So I put a spare battery on the floor in the back seat area in my truck cab and run a wire to the camper from there. This keeps me from drawing the main battery on the truck too low to start the engine. I also have a pair of solar panels which hang on the rear doors to keep the battery topped off while camping.
I have changed all the lights over to LEDs to reduce the power consumption while camping. I also installed a LED light strip from Ikea down the center of the camper ceiling... this lights the whole camper up real nice and draws very little power.
I've looked at that DIY plan before... it is actually different than a Bethany Pickupper layout.
I hope that helps...
Prybry