We were in Cody for a while three years ago. Knew nothing about the area but took a chance on the first place we came to. We stayed a Absaroka for 3 or 4 days. Price was right and folks were very nice. Paved sites and near downtown. Little too close together to call a resort, but the best thing was the vegetables. It was summer and the guy running the place had fresh tomatoes he shared with us. We would stay there again - if tomatoes are in season.
The park beside the reservoir is nice looking but I am not sure they have sewer.
The WalMart had a huge parking area for RVs and right after dark one night, we counted over 25 trailers, fivers and MHs. If no more than a couple of days stay, that might be an option