There's nothing wrong with putting antifreeze in the fresh water tank. Many manufacturers even list it as one of the ways to winterize your unit. I just wouldn't leave excess sitting in the tank after you run it through the lines. Drain the excess out through your low point drain if there is any. It should hurt with it in the lines of tank for 3-6 months. Run water through it afterwards until it's gone. If you are one that drinks from the water lines (we are not), then I would recommend sanitizing it at least once or twice.
With a reported smell, it sounds like water or something else has sat for a long period of time. With only 4000 miles on a 4 year old unit, I would imagine that it hasn't been used in a while.
