Presuming you drain the grey water with a garden style hose, you can use one of these.
Grey Water Drain Adapter
It slips into a rubber or foam seal then into the park sewer system.
Before I had one if these in my old pop up, I ran a short piece of hose into the drain and stuffed some paper towels into the drain to arrest the odor. When I left, the paper went down the drain and the end of the hose was cut off and put in the trash. Some RV parks will not let you use a hose the way I did so it is best to plan ahead