I think you might want to contact some of the cabin places and see if they will accommodate your RV as well (they won't have hook-ups, most likely, and they will probably charge you as if you are using a cabin - but try to negotiate with them, and maybe they'll let you park the RV on the same property). Here are a couple:
Have not stayed at either, but the properties always look well kept when driving through and they have business.
There is one combination cabin/full service campground place:
reviewed on TA here:
Notice that there aren't a lot of reviews - it's sort of secluded and I have never even peeked inside, but it sounds like what you're looking for. Note that some of the reviews bemoan hot weather and yellow jackets (neither will be a problem in December, so take those with a grain of salt).
All of Three Rivers is very...locally owned, and quaint. I like it a lot. It is much nicer in the winter than in the summer though.