I'm new here too, but your topic aroused my inner "wanna-fix-it". After purchasing our 2002 from a dear friend I too discovered the mysterious odor usually associated with the latrine after removing winterizing antifreeze from the potable water system. Draining and refilling did not make much difference, so we poured about 4 cups of bleach into the system, ran the water to fill all the lines and let it sit overnight.
Emptied the water and refilled the system a few times until the bleach odor was gone. In hind site I think we could have used less bleach, but all in all it worked out well for us.
Now... if this doesn't work, the odor may be coming from the coils in the hot water heater and the only remedy may be to remove them and clean or replace. A Camping World could do that repair if you don't feel up to tearing apart your hot water heater.
If the above frightens you, Camping World has a product that claims to sanitize your freshwater system. I have not used it, but maybe others have. Here's a link to their product page: