My 1994 PaceArrow didn't have a water heater by pass valve. I use to drain all tanks and hot water heater. Remove inlet side of water pump and attach hose. I had a 6gal water heater I would purchase two cases of antifreeze. Fill a 5gal bucket with the antifreeze and stick the water pump hose in bucket and keep filling as needed. Open up the farthest hot water fixture and start pumping when antifreeze comes out stop and go on to the next and then do the cold. When finished I would drain the water heater back into the gal jugs for next year. Every year after the first you only need a few gal.