Whenever John and I head up to Yakima from LV, we stay in Ely (258 miles from our house) at a pretty nice KOA...if you go, please say "Hi" to the burros for us. Then, we stay in Boise (you'll hit Jerome before then) the next night, which is 385 miles. Route 93 (using the 318 cuttoff mentioned by Dale) is a great road and an easy trip...just a little windy/hilly near Ely. The road is usually pretty empty.
To find the KOA, you'll take Rt 93 to 318, which will drop you onto Rt 6. When you get to Ely, you will literally run into Rts 50 & 93. Turn right at that intersection and go about 2 miles and you'll see the KOA on your right. When you leave the next morning, just turn left out of the park and go toward town. There is a signal light at the intersection where 93 heads north towards Idaho. From Ely to Jerome is about 270 miles according to my mapping software.
I agree that Wells is not really a good stopping point, except for fuel at the Flying J. Your next option would be Jackpot (on the NV/ID border), which is about 450 miles from LV...a little long, but again a fairly easy drive. There's a nice RV park at Cactus Pete's, but make reservations in the back part, as I believe if you just drive in they stick you in a different section closer to the casino parking lot. But then, your next day to Jerome would be a very short trip.
Sometimes, for a change, we will take Interstate 15, stay in Salt Lake City area the first night, they on to I-84 to Boise for night 2. Each of those legs is about 390 miles, depending where you stay in the Salt Lake area. As you can see, going the way of Rt 93 will save you about 100 miles or more.
Have a great trip!!
Gail, John & Mindy Stacy
2007 Alpine SE 40FDQS, 2011 Equinox
ACA & Alpine SoCal