You mentioned in your post that you will be visiting Rocky Mountain National Park. Just a heads up, there are no Flying J stations in the northern Colorado area that I'm aware of. Closest will be a Flying J and a Pilot in Cheyenne, WY near the I-25/I-80 interchange, or in the Denver/Aurora area off I-70.
There's Johnson's Corners Truck Stop in Loveland, south of I-25/US 34 interchange that is set up for RVs. There's also a Sam's Club on US 34 (Eisenhower Blvd), that's fairly easy to get in/out of. In Estes Park, if I need fuel I always use the Conoco station at the Country Market at Mary's Lake Rd and Moraine Ave. Easy in/out, but it can be a busy intersection. There's also a Shell station and a Safeway Fuel station on US 34 (Big Thompson Ave) as you're coming in to the business section of Estes Park on the north side of the street -- easy in/out for either. Both are next to each other by the Safeway and the intersection with US 36. Expect resort type prices in Estes Park.