No, don't try to fill the hot tank via the drain plug.
What I meant was that I personally pull the nylon plug (and open the pressure valve) on the heater, drain the heater tank. Then what I do is take a hose assembly I built out of hose and PVC fittings is a bypass for the fresh water pump. I disconnect the fresh pump from the fresh tank and I simply prime the line, the pump sucks in antifreeze right out of the antifreeze gallon jugs and while I am flowing it through the cold lines, and the hot lines, I then turn off the by-pass on the water heater and let some of the antifreeze enter the water heater tank.
I then button up the heater, replace the fresh pump to the fresh tank, place a bit of antifreeze into the fresh tank....and this is now all done after I hook my air compressor to the city water connection and blow out all the lines and heater tank.
In the spring, I hook up the Safari to the city water (with an inline filter in place) and flush the heck out of everything, then fill and drain the fresh tank a few times. I then place the right bleach per gallon mixutre into the fresh tank, flow that through all the plumbing and let it sit for the appropriate time. I then again flush the heck out of all plumbing and blow out all the remaining water in the plumbing with the compressor so that it's clean and ready for use. This seems to work well. I had the Bambi outdoor in below zero temps, and although the Safari hasn't really seen similar temps, it has also served this coach well too.
As John said, it's prob not required, but it also can't hurt and since I've already gone through all the trouble, it only takes a few extra minutes.
Your mileage may vary.