It is quite possible that the previous owners had let RV Antifreeze get into the water heater tank. This will definitely cause foul smelling water. Check to see if a water heater bypass is installed, and use it when winterizing.
Another cause is the water itself. High iron content water can contain concentrations of manganese and sulfur, and this too can cause stinky water. The manganese and sulfur interacts with the anode material, and gives off an odor.
What you will want to do is pour bleach in the FWT and fill with water to a half tank. Flush all the lines until you smell bleach, especially the faucet furthest from the water heater, this will let you know that tank is full of bleached water. Let it set for a day, then drain everything and fill the FWT with water. If you have an outdoor shower, flush the lines with that so you don't fill the gray tanks. Keep flushing until you no longer smell bleach. You may need to fill and drain the FWT a couple times before the smell goes away.
Doing this will not only clean the water heater tank, but disinfect all the lines too, which is recommended to be done occasionally.
I have an electric tank for my kitchen and ran into this problem. If the water softener ran out of salt, then iron water would get in the heater and stink up the place. I wound up installing a filter housing in the cold water line before the heater to clean the tank when it happened. I would pull the housing off, fill it half full of bleach, put it on and let the water run in the kitchen until I smelled bleach. Problem solved.