As your leaving Bozeman fill up at four corners. They usually have much cheaper fuel then West Yellowstone. The easiest station to get into with a big rig is the newly renovated Exon. As your coming south into town it will be on the left. All of the stations run the same prices for the most part, which is the highest in the area. One reason West has a city tax that others don't.
There is also a station on the right in Big Sky that has good prices, I don't remember the brand. I have stopped there several times.
Also stock up with grocery's in Bozeman, a lot cheaper and better quality. If you go to Safeway and get a rewards card you'll save money on grocery's and you can build fuel points.
For LP go to Granite Propane, the have the very best prices anywhere, it is also easy to get into. It will vary from when they get new delivery's. I filled a 20lber for $11.00 last Aug.
Most of the fuel stations inside the Park are accessible with a big rig with some maneuvering. I think some don't have diesel pumps.
You can look up prices on Gas Buddy .com
