DrewE wrote:
The check valve is typically built into the city water connection itself, a part of the hose connector assembly. There's no appreciable amount of water that gets trapped there. If you're still concerned, you could always pump antifreeze through the city water inlet.
Blowing out the lines with air is, of course, a perfectly valid approach to winterizing in general.
To the OP: If the hot water heater has been bypassed all year, then there's no need to do anything to it to winterize it--it can stay bypassed. It would be a good idea to remove the drain plug (if it was installed) to make sure there's no water in the tank when it comes time to winterize. The interior pipework does need winterization as always, though; not using the fresh water tank just means that it's not necessary to drain it for winterization, but no further reduction in the work involved.
The instructions for our RV state to push the center of the city water valve after the rest of the system is full of pink from using the water pump. The system is under pressure then and a bit of pink squirts out of the city water valve thus winterizing the entire line and valve.