I'll be interested in this as well. My assumptions would be as follows, hopefully someone here can verify:
Drain the water system for the camper (after turning off all pumps and removing any water supply of course). Remove the water filter from under the sink area.
On the A126 (assuming same on A122) there are drain valves under the sink that you PULL UP (do not turn or twist) to drain the system. Open the faucet valves while draining to ensure you get all the water out. You'll also have to open the fresh water tank valve under the camper (if the camper is low on the driver side, and nose down you'll get the most complete drain of that tank).
While the main systems is draining, open the cover for the water heater and use the appropriate sized wrench to remove the anode rod from the bottom of the hot water heater. This will allow the tank to drain. Again, having the camper low on the driver side will help the heater drain.
I haven't done this yet myself. Just going on what I remember from the delivery of the new camper. I also referenced this manual online - not sure if it's the exact model but I think its correct: http://www.lsrv.info/david/Suburban%...0REV%20new.pdf
The last step is to add RV Antifreeze to the fresh water tank and use the campers water pump to circulate that all the way through to the sink according to my dealer. This will take several gallons. I'm not sure that I would use enough to fill the hot water side of the system since at the smallest the heater is 4 gallons and there's no need to have the water heater full of RV antifreeze in my opinion.
Hopefully someone else can chime in here and give us some tips.
2011 FR Rockwood Premiere A126 Hard Side
TV=2011 Jeep Wrangler Sahara 2 Dr
Reese 66065 WDH, Curt WDH Shank #17120
Prodigy P2 Brake Controller, Mopar 7-pin Harness
B&M Transmission Cooler (#70268)