My 337DS has a bypass behind the water heater. You have to remove the inspection plates on the plastic water heater housing box to get to it. Drain the water heater and leave the plug out.
Getting the antifreeze in is easy with a pump from Camping world. I think mine was 19.95 last year. I found it easier and cheaper to blow out the system with air and just put AF in the sink drains. Black and gray tanks should be clean and empty and leave the dump valves open so the drain lines don't freeze.
John & Susan, Olivia the dachsaund
2011 Georgetown 337DS
Toad 2003 Dodge Dakota
Hampton Roads VA