A couple of strategies folks use, I've done both successfully. One, open all low point drains and allow water to drain. Remove the drain plug from the water heater (access outside, through cover) Turn water heater bypass to bypass (winterize) setting. See if your water system has a valve to allow suction by the pump to draw from RV antifreeze jugs. If so, stick the hose in the jug and turn on the pump. Go to each faucet and turn on cold faucet until pink antifreeze shows. Turn off, then turn on hot and do the same. Do this at each fixture in your RV, including shower and toilet. If you have a sprayer at the toilet, make sure it also runs 'pink'.
The other method is to blow the lines all empty with compressed air. You'll need a fitting to screw on the city water opening and hook an air line to the fitting (available at Amazon, Walmart, RV stores) Use less than 40 psi and pressurize system, opening faucets as above, blowing until only air comes through the lines. You'll still need a gallon of RV antifreeze to fill the drain traps to prevent them from freezing. Many videos and "How to" on the interwebs, just Google to find more detailed instructions.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI