I take it you have a full hook up site if so run water till the tank is full then leave 1-2 faucets running and go out and pull the dump handle and let the tank empty and leave the handle open for a bit the same way you do with a black tank flush. Then refill the gray tank again but add about a pint of bleach as you fill the tank and let this set overnight. Then when emptying it again leave a couple faucets running inside to act like the black tank flush.
Then ib the future try to prevent ANY food waste from going down the sink drain, this is where your nasty smell is coming from is the food waste. You can't just rinse the food scraps down the drain like you do at home.