Your profile says you are from NE PA. Not knowing where exactly I assumed maybe somewhere around Meadville. That's 1800 miles to Yellowstone east entrance. We just finished a 3 week 5200 mile trip and averaged 50 mph with fuel stops, toilet breaks and lunch breaks. 1800 miles will take you about 36 hours of steady hard driving. We did one day of 14 hours and had to spend the next day recovering. All other days were 8-10 hours.
I drove through Fishing Bridge park just to check it out and DO NOT recommend it as they pack RV's in there like sardines in a can. Not for me but maybe you won't mind it. We went to Yellowstone, I recommend staying in West Yellowstone. It's right at the west entrance and is a nice small town with everything you need. We stayed at the Buffalo Crossing RV Park. All we needed was within walking distance.
Hope this helps.
