Hwy 99 is just fine and there are plenty of places to stop and take a break. The rest stop in Turlock is open, but rest stops aren't my first choice. You have a nice Camping World in Bakersfield and another in Fresno. Both good places to take a break and look around at RV supplies. Every town has a Walmart or a Target or both to stop at. There is a Walmart Super Center in Atwater. If you like air museums there is also a great one of those at the old Castle Air Force Base in Atwater. Just follow the signs, it's a few miles off the freeway.
If you're staying at the Lakes RV Park in Chowchilla, it's a very nice little park and each night stay used to come with a round of golf for two, I assume they still do that. You've got other RV parks on your route but you'll probably only need the one if your main goal is Sacramento.