Mikla, as Pierre said I prefer campgrounds to dry camping, though I did my share of that too.
I now have my second 5th wheel trailer since 1988, both of which I have used extensively every Winter for skiing in NM, Colorado and Utah. Both needed quite a bit of work to make them ready for 15 below nights which are often encountered in Jan.and Feb.
They both had the prerequisite factory stuff like storm windows (1/4 glass in the Jayco and framed 1/8 plastic in my present Carri-Lite) and closed underbelly with insulation between the holding tanks and the outside world.
Nevertheless, The space in the Jayco holding tank area was not heated and I went to Camping World and got four self stick electric heating pads which work on 12/120volts, two per tank and stuck them underneath the tanks, they worked fine and I didn't have a freezup in the tanks but the dump valves and the dump pipe froze up and I eventually used electric heater tape to wrap around and that helped somewhat. No problem with the water tank since it was under the sofa in the living area. I could keep the trailer warm for the most part with just one electric heater. Forget the heater in the AC, it is just for cool Summer nights.
In 1998 when I bought my new Carri-Lite, also 28 ft 5th wheel with a 14 ft slide out, which makes the interior space quite large, I made sure I had more of the things I needed for Winter camping but I still had to a number of things to prepare it.
It has a pass-through basement which houses the water tank, it is a large space where we keep the skis and stuff and it has a register from the propane heater, and also a sub-basement which hold the holding tanks and which is heated via piped furnace heat, so the propane furnace needs to be used and in very cold weather I go through ten bucks worth of propane every three days, not too shabby when you consider the cost of lift tickets.
I bought an indoor/outdoor thermometer and snaked the outdoor sensor into the basement which holds the water tank so I can read the temperature there from the inside of the coach. I also installed a lamp recepticle in that space and if it gets REAL cold I use a 100 or 200 watt bulb and leave it on all night, that keeps the temps up.
Never leave your water hose or dump hose connected to the trailer. Use the water out of your tank and when it gets low, fill it and drain the hose afterward and stow it away. The same with the sewer hose, otherwise it will break into a spiral wire mess.
I tow with a 3/4 ton Dodge Cummins Turbo Diesel and have no problem with pulling the 10,000 pound trailer over the continental divide. It has 4-wheel drive which I use a lot out there. I have a plug on it which keeps the oil warm and I use it all the time.
I prefer campgrounds because of the amenities. Tiger Run in Breckenridge has a large indoor swimming pool, two hot tubs, excellent shower and toilet facilities and cable at the camp site. Skiing at Aspen or Snowmass, the KOA in Basalt is only 20 minutes away. If you turn right over the bridge from the I-70 exit to Park City and the Canyons there is a campground right there which is open in Winter as is one in Sandy at the ouskirts of Salt Lake City, a half our from six Utah resorts.
Well anyway, any other questions, ask and Pierre or I will try to answer them.