Just plain old good manners are all that are required to fuel at the truck-pumps; that is something that is completely lacking at the car-pumps; i.e. people darting around and cutting others off or leaving their car blocking the pumps while they attend to lengthy morning duties.
Usually, via the phone hanging at the pump, the fuel-desk girl will ask for company name, state, plate number, hub reading, and how you intend to pay.
Just answer "private owner" as company name.
I have ALWAYS received a considerable cash discount for paying with cash, often as much as 10%.
They don't advertise this as it is illegal to charge less for cash transactions; but, it costs them a ton of money to process credit-cards, so they pass the savings along to those who are respectful enough to use cash.
I have found this to be also true at most motels/hotels; cash always gets a big discount.
Once you are done fueling and BEFORE you go in to pay, pull forward enough to clear the pumps, but don't go hide around back just yet.
Then, go inside to the FUEL DESK, NOT where common tourists are paying for candy-bars and souvenirs.
If you buy enough fuel, you are elligible for a FREE shower; if you need a shower, let them know.
Also, at the good stops that actually have a real restaurant, a minimum fuel purchase often nets you a free buffet or meal-ticket; ask, as they may not mention it if you don't.