When it comes to winter camping with full utilities (water tank, hot water, non-winterized gray and black tanks) it is always good to be conservative at first until you get some experience with your own camper.
We bought several wireless thermometers and placed them in various parts of our winterized camper to determine how cold those areas got, prior to running with full utilities. We installed 3-way valves in the plumbing to the outside shower and the city water line so they could be isolated and drained, while still having water everywhere else. Finally the location of your gray and black tank and their valves are another consideration. Ours are in a heated basement.
Weather in the Sierra in December has a large range from cold with snow levels in the foothills (~1,500 ft) or it can be warm and raining at 9,000 feet. It just depends. Always better to have data than opinions.
If you have time, you might make a trial run to Mountain High Ski area out side of Wrightwood, CA this weekend to test things out. Their North or West mountain base areas are around 7,000 feet and it is supposed to be snowy this weekend and then warm up again.
After that I'm sure you will come up with a long list of places to insulate / cover / seal to make it more enjoyable, so you can stay out longer. We can stay in one place about 5 days before we run out of water / gray tank capacity, and go through 20 lbs of propane while keeping the hot water tank running and the inside at 70 ยฐF.
'05 Ram 3500, 4x4, DRW, LB, 6spd man, CTD, PRXB exhaust brake, Roadmaster bar
'01 Corsair 10'8" - 4,200 lbs., Xantrex XADC 80A, Link 20, 4-Lifeline GPL-4CT, PowerGate Isolater, 2 AWG wire, PI 30A EMS, 2 Honda EU2000i, parallel kit, ext. duration tank.