So Cal to Yellowstone

We make that trip a couple of times a year. It's hard to avoid the I-15 so we push hard the first day to get to cooler temperatures in the higher elevations of Utah. St. George is a nice town but still pretty warm so we push on to Beaver, Utah. There are a couple of nice RV parks there. It's about 500 miles from our house but gets the worst part of the trip out of the way. You can stop earlier at say Cedar City if you choose and still be in a cooler climate for the night. The next day through Salt Lake City turning onto the 89 at Brigham City. From there through Logan and Logan Canyon for a scenic way, stopping overnight at Rendevous Beach campground at Garden City, Ut. The next day you travel along the shore of Bear Lake into Idaho and Wyoming, then drive through the Star Valley and along the Snake River into Jackson, Wy. If you haven't done the Tetons (Yellowstone's southern neighbor) you might want to give it a look. Has an abundance of wildlife plus those magic mountain scenes. This route will get you into Jackson about noon where you can continue up to Yellowstone through the southern entrance if you don't want to explore the Tetons.
If you're in more of a hurry, the I-15 to the Idaho 20 will get you into West Yellowstone the fastest.