If you remove the drain plug from the water heater, then open the hot faucet at the kitchen sink, you should drain most all the water from the tank. When the water has stopped draining, you can install a cap like this on your city water female connection. While buying the blow out plug, you could also order a water heater winterizing kit and install it for the future if you can't locate one already installed.
Using an air compressor set at no more than about 40 psi, you can go from faucet to faucet opening the cold until you get air, close it, then open the hot. Do this at each sink, shower, and the toilet, including the sprayer. (if equipped) Don't forget the outside shower. Repeat to see if any water has run back to a low spot and needs a second blow out. Add RV anti-freeze to each trap. If you have an ice maker or washer hook up, blow them out also. After you have blown out all plumbing, open all low point drains and remove or loosen the water heater drain plug.