Google Maps/Google Earth & Gas Buddy are your best tools. I'm a Planner so I am researching Maps before longer trips to ease the stress of looking for Fuel. Since fuel consumption in somewhat static I tend to look xx miles ahead so I know approximately where to look online. DW is more 'Serendipity' (hence our name
) so I have DW on her smart Phone verifying ahead when we get to 1/2 tank. Tends to work for us. Multiple Apps for Android, IPhone can also help.
Old School but someone gave us a book called "The Next Exit" which lists RV friendly Stations in red along Major Highways only.
Rented a Class C in Alaska in 2013. Best advice came from the Driver taking us to the Rental place. He said always fill when you get to 1/2 or 1/3 because you do not always know if the station is still there or possibly closed. Alaska is more extreme but I believe it also holds true in the Lower 48