re: Yellowstone

It's been several years since I've been, so my recs are likely dated, but I will say if you're thinking about it now, book very soon. It wouldn't surprise me if all the hotels in the park are already booked for the summer. Yellowstone is VERY popular and a lot of people book up to a year in advance. RV spots may be even more difficult.

There are a lot outside the park though, including a lot of VRBO's. People fly into either Salt Lake City or Jackson Hole. Jcakson is closer, but flights are pricier. I think SLC is about a 3-4 hour drive.