To answer your original question, you can use compressed air to blow out your water line to keep them from freezing so you don't have to use antifreeze. You can put a little antifreeze in the traps to keep them from freezing in your bath and kitchen area. I'm wondering if you have small water drains under your RV where you can at least drain the fresh water out of the tank.
I know your main concern was snow and keeping the slide pulled in; but you might need to make some arrangement to have a large stand alone propane tank set up by your 5th wheel so you will have good supply for heat. You can also use light bulbs as heat sources in the bay areas. Have you checked to make sure the campground will have water turned on? Are they really providing full hook ups for the winter? I live in Kernville and at the moment we are in Bridgeport, CA. We had hoped to spend a few days in Mammoth and we found the campgrounds with full hook ups are closing or will be closed the end of this month.
However if you want to try it, here are some suggestions:
*Use only the campground facilities ie showers & bathrooms if they are open.
*use only bottled water and not your tank water
*set your furnace to 50 degrees and open all cabinets so that the heat circulates. If you have drained all your lines, you might not need to do this.
Now the snow is another matter. It sounds like the campground regularly takes care of other peoples RVs, so maybe they have a technique that works and doesn't do any damage to your rig or the ones doing it.....
As Lou mentioned, I'd be very concerned with leaving a rig unattended with things running.