We had no problems finding diesel, even in Death Valley when we were in CA a couple years ago. Any area that has a lot of RV traffic (parks ect) will usually have diesel at the fuel stations. If you are near a major highway that is. If you are out in a sparsely populated area you can run into problems. Gas Buddy is the best solution and if you are in an area that you think you might have a problem then fill up when your tank gets to half full.
That being said, be wary of stations that may not get much business, diesel does not improve while sitting in an under ground tank. It absorbs water from the air.