Definitely put water in the traps and check them out. Gray water can stink so bad!! I was surprised at how bad the gray can smell. We had that problem ONCE! Now we put a little water in the traps each time we dump. We add Odo-Ban to the water and have never had another problem with smells. Odo-Ban is a Eucalyptus scented disinfectant/cleaner that is available in gallon bottles from Home Depot in the cleaning supplies department.
I too use the Flush King. It allows me to fill the tanks full before dumping without using a wand. It also makes it easy to back fill them with fresh water to flush them clean. It does take some time to fill and dump two 50 gallon tanks though. If there is a line at the dump station I skip the backfill/flush step, but seldom have to.