There is a KOA on I-15 on the north side of Barstow, and another one called Shady Lane RV park in the same area. You can find info and directions on
www.woodalls.com. We haven't stayed at either, but they are close to town and had good reviews.
We have overnighted at Twin Lakes in Newberry Springs, just east of Barstow on I-40. However, it was very deserted and remote and we plan to go to one of the other parks when we pass through that area again.