I'm finishing up an 11 week trip, and have found the best prices by far for diesel are at Indian casinos. I topped off my tank at Feather Falls casino in Oroville Ca for $3.39 today, and at the casino in Ukiah yesterday for $3.39 yesterday. Both were #2 diesel and at RV/truck islands. Most other stations in the area are around or over $4 a gallon and pumping B20 biodiesel. I had similar good fortune in Oregon and Washington.
I always check at gasprices.com before I buy diesel and plan my trips accordingly. The 2 states you want to avoid having to buy in are CA & WA.
http://www.oregongasprices.com/index.aspx?s=Y&fuel=D&area=All+Areas&station=All+Stations&tme_limit=48&site=Oregon&srch=1&list=0
The Casino takes about 10 minutes to get to off the highway but it's much less expensive than anywhere else in the state. Nice RV park there and super modern and clean pump islands and huge store. We stop there any time heading down into the valley and then north. Talked to a trucker who said it saves his company $4K/mo. over other stations.
Carson City, Nevada is another good diesel stop. We just purchased diesel both ways on our 395 trip at the end of July for $3.09. Gas is $2.99. Check GasBuddy for latest prices. Station is at the south end of town.
I always buy B20 or higher if the price is competitive. Most of the diesel down in the valley is a percentage of biodiesel.
Pierce
This is where I bought diesel in Washington. I always look for a nearby casino.