I don't worry about any water left in the lines during the camping season and only clear the lines when it's time to winterize. The only way I know to clear the water out of the lines is to use a air compressor and adapter that connects to the city water connector. Turning on the sink won't flush the lines and opening the low point drains doesn't completely clear them either. I also tend to leave the water in the hot water heater between trips.
2017 Ford F250 XLT, 6.2 gasser
2013 26BH (traded)