We are going to Fishing Bridge this summer. We have reserved a full hookup site so there is no need for a gen. I am not sure you will need AC either. The weather varies wildly in Yellowstone lately. But normally, given the altitude, the nights are cool to cold. When we tented at Fishing Bridge (when you were allowed to years ago), we used a Coleman heater every night. Last trip a few years ago, it was in the high 90's every day and the nights were warm. Since you can't use a generator at night, A/C isn't going to be an option without a hookup. We have a 50 amp/water/sewage site. You may want to see if you can get one too.