If you are not towing with an RV that size I would guess the GasBuddy app would be perfect for you. You can configure it just for showing you stations that have diesel.
However, if you are towing and want to ensure you never are in a congested situation where you have to back up, why not just use Pilot and Flying J. I got their card through Good Sams when we were full time in our 5th wheel and used them exclusively. You go through the semi diesel lanes and get a pretty good discount per gallon. Since you fuel up where the big trucks are you never have to worry about getting in a tight situation or old/bad fuel. They also have their own app that has a trip planner. I still use the trip planner on my I-phone even though I don't use diesel anymore.
Full Time in 2017 Newmar Ventana 4369
pulling a 24' enclosed (Mini Cooper/Harley/Kayak)
(traded 2014 Thor Challenger)
US Army 70-73 Retired LEO