Eric,
Drive away with confidence. With family in central Oregon we make frequent trips north. When we had diesels we never had any trouble finding fuel. Larry and the others are correct about finding fuel along major interstates. Texaco, for one almost always has diesel. Since Chevron and Texaco have merged your chances are even greater of finding diesel. But just in case email your concern directly to them and they will send you a list of stations along your route AND in N. California:
email address:
texacoweb@chevrontexaco.com
You are not going to be stranded unless you simply ignore your fuel gauge, then it wouldn't make any difference whether it is diesel or gas.
Regarding N. California, specifically San Jose area, just look for a Rotten Robbie stations. They are all over and pump huge volumes of fuel at discounted prices.
Haasman