We did stay a few days in SLC and found it to be overpriced for what you got. As most RV Parks in the Yellowstone area are "first come - first served" we over-nighted where an early departure would arrive in the park @ mid morning. It was Jessen's RV Park located 2 miles south of Ashton on Hwy. 20. Nice little widow lady and the view of the Tetons some 50 miles away was great.
Her card has a phone - 800-747-3356 and internet site - www.jessenrv.com
neither of which have we tried.
They will be on your side of the road (east) and should you miss it (we did) there is a truck turnaround a 1/2 mile further.