We travel mostly the "blue roads" and have never had problems finding diesel...it's a rare small town that doesn't have at least one diesel pump.
You can check sites such as
this for stations along the way that have diesel.
Sites like DieselBoss have a listing of truck stops, but they're mostly along the main highways. By the same token, books such as
The Next Exit and
The RVer's Friend list gas stations, but mainly along the main highways.