If you're looking for something that is a couple hours South of Sac, there is a town just off I-5 called Santa Nella. Take off the interstate and go LEFT. There are several gas stations, motels, and restaurants (incl. Starbucks). I think most of I-5 is pretty bare and desolate so I don't know of any other places to stop other than Harris Ranch.
I live about 2 hours south of you as well (off 99 around the Merced area). I don't think you'd need to fill up so quickly but just so you know, there is a Costco right on the highway in Modesto, Turlock and Merced.
Fresno would be about 3 hours south of Sac and it's a big town, so there are lots of places to stop there. After Fresno, probably the next decent sized town would be Hanford and then Tulare (home of the famous Tulare Farm Show!). Tulare would probably be about 4 hours into your trip. After that, it's mostly dusty little farm towns until you get into Bakersfield.
When we drive down, we usually stop around the Magic Mountain area in Santa Clarita. It's about 4 hours-ish from me, so add another 2 hours for you. It gets us over a good portion of the Grapevine and we always time it to stop for dinner at Marie Calendars and miss the LA rush hour traffic.
Hope this info helps you. Not sure if I answered your question directly but maybe you can find some tidbit of info that will be helpful!