Yes, purchase the Mountain Directory. We use it all the time.
You really didn't clarify if Yellowstone is to be your only destination or are you up to more? To get you started, get on I-15 heading to Las Vegas. You could easily stop at Zion Nat'l Park taking Hwy 9 around St. George, Utah. Stay in Zion itself or in nearby Springdale. The free shuttle makes runs in this neat little town taking you back into the park. You could also do a day trip to Grand Canyon - North Rim from the Zion area - or even drive up there and stay.
From Zion go back over to I-15 heading north. If you want to work in Bryce Nat'l Park take Hwy 20 over to 89 and a little south to Hwy 12 to Bryce. Stay in the park or just outside at Ruby's where again, the free shuttle runs for the park.
From Bryce you could drive further on Hwy 12 for a awesome day trip. You probably won't want to take the RV further on 12. To Bryce it's very easy. Further on 12 is Capitol Reef Nat'l Park but with the trailer you can get to it an easier way.
So...from Bryce backtrack a little to Hwy 89 north to I-70 NE and pick up 24 which is a easier way to Capitol Reef.
Hwy 70 east will also take you to Arches Nat'l Park on UT191, if desired. Don't know how much time you have for this trip. Pick and choose.
The easiest way without big grades heading north again, depending where you are at this point, would be to take I-15 into Idaho. If you're over by Arches, then Hwy 6/191 at Green River and I-70 is o.k., taking you into Spanish Fork on I-15 just south of Salt Lake City.
Now you're in Idaho. At Idaho Falls pick up Hwy 26 a flat lovely drive along the river taking you to Alpine, Wyoming. Get on 89 north to Jackson (a couple big groceries just before the town of Jackson (Smith's on the left and Albertson's, I believe, on the right). At Smith's there's also a laundry, good fuel prices and a liquor store. Then continue through Jackson and into Grand Teton Nat'l Park. Stay IN the park for the best experience. If you need full hookups then Colter Bay RV park is the best. If you can do without hookups, we prefer Gros Ventre or Colter Bay campground (next to the above-mentioned RV park), and both are in the park. Those 2 dry campgrounds don't accept reservations but they both have 300 sites and rarely fill except perhaps on July 4 weekend.
Then you can drive north directly into Yellowstone. Fishing Bridge RV in the park is the only one for hookups and there are many dry campgrounds in the park. Madison is a nice one. If staying outside the park, West Yellowstone has the easiest access and there are many RV parks around that area. Remember, Yellowstone is huge so you don't want to stay far away. You'll be driving in the park all day long for multiple days to see the whole park.
For each national park you'll be visiting it is advised to check out each official national park site for those parks. They have tons of information, what to do in the park, campground and driving information and road updates. For instance, here is Yellowstone's:
YellowstoneHave fun planning your trip!
Full-Timed for 16 Years
.... Back in S&B Again
Traveled 8 yr in a 40' 2004 Newmar Dutch Star Motorhome
& 8 yr in a 33' Travel Supreme 5th Wheel