As Flying Driver says, when you make up a DOT number, you are billing some trucker for the fuel tax on your fuel. It isn't coming out the the state's coffers - they WILL collect their tax and they will get it form whomever has that number.
The only real issue is that the advertised price for the truck pumps is without tax. Truckers pay the tax quarterly, whereas cars pay it "at the pump". A private motorhome pays the same price as a car, which is a lot higher than what you saw on the roadside sign. Check the auto diesel p;umps and you will see - you were not being "ripped off".
By the way, at the truck pumps you insert the Pilot/FJ RV card first. That should turn on the pump and recognize you as a non-commercial vehicle and set the pump price accordingly. Then you pay later with your credit card or cash.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL