[JRTJH]

The easiest way to flush the water heater and not disturb any of the other plumbing in the trailer would be to remove the anode rod, place a cut end of a 1/2" hose into the anode opening and run fresh water until it runs clear. I'd probably flush it for about 5 minutes to be sure all the antifreeze is out of the water heater tank. Then, wrap the anode rod threads with fresh Teflon tape and reinstall. Don't forget to close the bypass valve on the back of the water heater or when you flush your plumbing lines in the spring, all of the cold water line antifreeze that's "ahead of the water heater inlet" will divert through the water heater and you'll have problems running enough water to flush all of that antifreeze out of the water heater tank.

With the bypass valve in the winterize position, water will divert to the hot water lines and not enter the water heater. That will make it much easier to flush all the antifreeze out of the lines in the spring.

If I were you, and the lines are winterized, I'd just isolate the water heater (by closing the bypass valve). I wouldn't "reinvent winterizing" and redo all of what's already done......

The bypass valve is located on the back of the water heater, on the lower (inlet) fitting. It is a 1/4 turn fitting. One position diverts water INTO the water heater and turning it 1/4 turn diverts water AWAY from the water heater and back into the hot water lines. There is a one way valve in the top fitting (outlet) from the water heater, so water will not backflow into the tank.

Here is a picture of the back of a typical water heater, the bypass valve is in the lower fitting that is at the extreme bottom of the upright tube that's just to the right of the water heater inlet fitting.