Interesting thread, here is my pespective based on:
- growing up in a downhill sking family in Canada, using a Class C Vanguard 24' almost every weekend, temps down below -40F/C regularly.
- currently owning a Bigfoot Class C 26', using it more in the winter than the summer, coldest temps down to -45 F/C.
First - keep it simple, why even try to keep pressurized water going at those temps. I winterize the lines every fall and then for winter use a 10 gal portable water container. Still use the grey/black tanks since they are in the heated basement. If it freezes up big deal, it freezes from the bottom, lots of room to expand and eventually it warms up/thaws and you dump. Biggest problem are the dump valves. Also solids/paper are not your friend, winter is liquids only in the tanks.
Second - electric heat is not the way to go, keep your propane heat going is the key. This way the heat is distributed in the unit like it was designed to do, using portable electic heat as your primary heat source just keeps the propane thermostat from coming on when it needs to. I use a 1500W heater only in the cold metal cab when plugged in to keep condensation/frost down.
Third - how to keep propane flowing @-40F/C, secret is a Honda generator. I use only a EU2000i, small enough to keep inside and warm so it will start, large enough to run a small heater I can direct at the propane tank or use for the block heater to start the engine in the morning. (I don't have a built in generator, probably wouldn't start being so cold. Generator is also enough to give a bit of charge to the batteries elsewise.
Fourth - massive batteries, battery performance in cold is exponetially worse, below -40F/C the propane heat will cycle over 75% duty, this is a big draw for the fan plus all the other ancillary load. I have two 210 amp hour Lifeline AGM deep cycle batteries. Plus all interior lighting is LED (Command Electronic). All charging wiring upgraded plus marine quality battery monitor that measure amps/voltage/amphours and capacity info for both house and engine batteries. Also have an AGM for the engine starting battery, they will not freeze if you run them dead.
Fifth - insulation, all roof vents have removable inserts (venting when cooking to get rid of water vapour is important), custom insulated front windshield cover (better than using a blanket to block the cab, less condensation), heat shrunk window film if not double pane windows.
Sixth - condensation is a problem, even with proper venting while cooking, walls/windows will frost up. Air circulation is the key, especially when driving, this is the time to get all bedding/mattereses away from walls and blow the cabin heat as much as possible, the Bigfoot also has an rear engine coolant heater which is great when driving to keep the heat in the back.
Notice none of these measures are specific to the motorhome, any unit that has some kind of heated tanks can do all of the above. Trying to fix something or get something started in the very cold is not fun. My two best friends are a propane soldering torch and the Honda generator, as long as I can get one of them going, I can get everything else working.
Bigfoot before they stopped making Class C made an amazing winter motorhome but the 1980's era Vanguard is still used today in the same conditions and does just as well. In some ways the Vanguard is better. You can access everywhere to get to wiring, hoses, tanks, etc. and the systems are simple to figure out and fix.
Oh and don't run out of propane, I have a spare tank with an tee adapter on the main internal tank so I can tap in just in case. Comes in handy too when it is so cold the local propane distributor is frozen up or your tank fill is iced up (it happens...)
A fast permanent fix is this: www.horstmiracleprobes.com
I put a set of the grey and black water probes in two years ago and no problems since. Easy installation as long as you can see the sides of your tank. Leave the existing probes in place, drill in the new ones and move the wires.