I got these 'tablet' deodorant things that you dissolve in the sink that goes to the grey tank. I put one of these down the drain after I dump.
By the next time I need to dump with a full grey tank, it's not near as foul smelling as it was before...
The first time I treated my tank this way, I filled the tank with about 10 gallons of water (30 gallon tank) before I left home. The guy at the RV store where I got the deodorizer suggested this to kinda 'stir up' the tank and it'll get rid of the crud build up on the walls of the tank and whatnot..
That was a couple of years ago and my grey tank does not smell as bad as it did before, and the tank sensors actually work better too! :)
Good luck!
Mitch