For Yellowstone:
'Projected' Road openings (depending on the weather) is from the north entrance at Mammoth to Old Faithful is April 15 and openings are then staggered to May 27 when they're completely open.
For campgrounds in the park, Mammoth is open all year; Madison open April 29 and Fishing Bridge RV Park open May 6. Others follow.
Here is the park's official web site with all this information and more. Click on every link and refer to it often before your trip for any updates on closures/maintenance, etc.
By the way, each national park has a site like this and they are well-worth studying before a trip.
Yellowstone Nat'l ParkAlso, with your small RV and time of year, don't make reservations. You'll get sites and then you can drive at leisure, stopping where you want and when you want. We rarely made reservations even with our 40' motorhome and we got into Yellowstone and other national parks.
Definitely add Grand Teton National Park since it's right next door to Yellowstone. It's completely different in looks and things to do. Think of Banff. :)
Grand Teton National Park
