We had our first RV trip last March we're hooked. After a slow start getting used to driving the RV we had a fantastic holiday. I'm planning a trip next May leaving from Denver and heading up through the Rocky Mountain NP, Custer State Park, Bighorn Canyon and Yellowstone. Then south through Utah to Moab, Mesa Verde and Durango before returning to Denver. I'd like to know what Yellowstone will be like in late May. I've heard it can get pretty busy. Should I book a campground in advance or just turn up? Also, we've planned an 18 day vacation and don't mind a couple of days of hard driving. Is it realistic to think we can include Glacier NP in the trip? Thanks in advance.