I would go with gray water tank if everything is normal. Grease and other things like to stay in the tank and rot, if not flushed adequately and even then.
You're not leaving power vents open, I assume.
We use chemicals to control possibility of stink.
When parked, we sometimes leave the gray water open when washing clothes and there can be quite a bit ,of pressure back up into tanks, we've discovered, in some RV Parks. The ones that don't have field lines septic systems, just tanks, that have to be pumped usually. Our gray water smells like like liquid/gel bath soap and shampoo or laundry soap/bleach. Most of the time, after washing clothes and any time we leave the coach we close the tank valve, not only scents but critters like slugs, we discovered in Mississippi.
Put detergent and water softener into the tanks, partially full. Next trip down the road should/may clear out any deposits when you drain and flush the tanks if that is the problem.