The pressurized air will blow the water straight up the vertical pipe runs, so parking on a hill won't effect that. I winterize with air every year.
As someone mentioned, it might mess up draining the waste/fresh water tanks though.
Or, where you're located, get a small personal heater (200-250W) and put it in the bay with the water pump and gate valves and plug it in at night for the next couple of days. That's what I'm gonna do...not quite ready to give up on the camping season yet! LOL I'm hoping for one more trip right after Thanksgiving. :)
~Rick
