Way too many people use just water and have no problems with smell. Others, myself included, just use a little laundry soap and water softener, again with no smell issues. I'm betting on venting issue or you have a trap that's dry in one of the sinks and it's actually gray water tank smell (Bathroom Sink, Shower or Kitchen Sink). Lots of people have this issue when they don't use their showers and use the campground facilities.
Try to see if it is an all the time thing or only when the tank is half full or more or nearly completely full. Narrowing down when you start to notice the smell will help in figuring out if it is a tank vent issue mentioned above.
I will bet that if your using the walmart treatment and you switch to their more expensive treatment, it's not going to solve the issue.
