In a related situation, I'm replumbing a Fleetwood, multiple burst CPVC pipes with PEX-A .
The water heater bypass is a bit of a mystery, the bypass valve is on top and the cold feed comes up from the floor to the bypass and then back down into the tank cold inlet. Likewise the hot outlet goes up to the bypass and then feeds down to the floor. So this tank can't drain unless the drain plug is removed. I'm replumbing it now, why not run from the floor to the lower tank inlet, remove the whole bypass assembly and have the upper hot water outlet go straight to the floor and there is a T to the sink,shower, kitchen and the other side goes to a drain in the rear locker, same for the cold.
I plan on opening everything and draining as much as will drain at the rear, then blowing out the system with air from the kitchen, which is the far end of the system, Hot first, then close off hot shower, sink and rear drain and blow out cold pipes. This should get everything but the last bit of water in the heater that is below the cold inlet. If I do use anti-freeze, I would add it from the kitchen tap, only a 1qt or so on hot and cold and again blow most of that out with air, just to get the one low spot between kitchen and bathroom where pipes go under the floor and back up under vanity. No bypass needed and it simplifies the plumbing by the heater.