I was searching the Internet to try and figure out what the best current diesel price in the US is and stumbled on
http://www.findfuelstops.com. You put in your start location, destination and via if applicable. For Driving you would select 'RV', and you have options on currency and measurements (gal/liters miles/kilometers). Put in your starting fuel range, your full tank fuel range, select how you want the results sorted and select desired amenities (optional). You can even add filters for stations. They even have
Fuel Finder™ Lite (beta): Designed for mobile devices.