We are headed out to the Quarryville, PA Jellystone this weekend and from what I've seen the predicted temps to be, you shouldn't have an issue in the Central PA area.
Flush your system out then sanitize it by adding 1/4 cup of bleach for each 15 gallons of fresh water capacity to the tank. After you fill up the tank with water, run the solution through all the outlets until you smell bleach. Let sit for an hour or so then drain the tank refill and flush until the bleach smell is gone.
As for the the Hot Water Heater, check the water line feeding it, there may be a bypass valve (isolates the heater so you don't need antifreeze) if there is you will need to open it to get water into the heater. After you get water in the heater run a hot water faucet until you don't get anymore air, then turn on the heater. Don't turn on the heater without water in it.
I'll second what Quetse said, disconnect the water supply hose and drain it if it is going below freezing. We ordered our coach with fresh, grey, and black tank heaters to extend our season, but there isn't much you can do about the supply hose freezing.
2004 Dodge Ram 2500 Club Cab 4x4 CTD
2007 Sunline Solaris T-276-SR: Misty Blue/Cherry
2001 Dodge Ram 1500 Reg Cab 4x4 OFF-ROAD