As I alluded to in John's (Losangeles) current thread on Yosemite, my wife and I went into Upper Pines Campground almost 15 years ago over New Years weekend. This is the only campground open in the winter in the Valley and like the other two Pines campgrounds it does not have electrical. That was our second trip in our new truck camper. There were several inches of old snow on the ground during our stay, but the skies remained a brilliant blue the whole weekend. Our biggest mistake was not having a source to keep our one battery charged to run the heater. A 75 watt solar panel was not nearly enough for 3 nights. Plus, Half Dome provided an unanticipated shadow for several extra mid morning hours during which the solar panel didn't get charged. The campground is well treed so solar can't be counted upon in any case. Although we seldom use the generator we now carry, a generator to power your batteries during generator hours would be prudent. Otherwise, you could end up limiting time running your heater.
Regarding weather, the Valley is at 4000'. In January highs average in the high 40s, nighttime lows average in the high 20s, and 6" plus precip, probably mostly in the form of rain. Pay attention to your weather forecasts. Worst case of an Artic Storm or a cold snap, it is only a 2 hour drive or so to get out of it. The Village had cell reception years ago so it should be pretty good by now. Don't know if the shuttle buses run in the winter.
If I were going this year in January in our 5.0TA, I would take our generator, and do nothing special to the tanks other than have some anti freeze available to use. Foam insulation and heat pads (during generator time) would be a plus. There are adequate bathrooms throughout the campground with flushies, and cold running water sinks. I believe the outdoor running water stations normally run in the winter. There is a dump station at Upper Pines to use on the way out. If you have a five or ten gallon blue boy you could even readily do some interim gray water dumping there.
Good luck, Cliff