I am somewhat familiar with the area as I lived most of my life east of Harrisburg until I became a fulltimer in 2001. I'm sure someone else will be able to offer better info but I will tell you what I know.
These are supposed to be open all year: the Harrisburg East CG and the Hershey Highmeadow CG. I have not stayed in either although I have bee to the Hershey one. It does not have water at the sites during Jan-Mar I know. The website for the Hbg East is
hereand for Hershey is
here
Ar for weather, I've not been there in the winter in a long time, except at Christmas, but my family stills lives there. The winters can be mild with little snow or they can be quite miserable with plenty of cold, snow and ice. It rarely snows before Jan. Personally I would never want to spend the winter months there in an RV if I had a choice.
About five years ago I drove the coach there for the Christmas holiday because I had to have some medical tests and treatments afterwards. I ended up being there 5 weeks and was going through a 28 gal tank of propane a week just to keep the coach warm enough to keep things from freezing. That happened to be in the top 5 for coldest Jan's or record though. I've been there for Christmas since when I never had to wear a coat.
Good luck with it. I sure don't envy you.