In general, when you get fuel at the truck islands you do have to go inside and talk to the teller. You may have to leave a credit card or pre pay to get fuel. Oh yes, it is a good idea to get the pump number you are at as they usually want to know that. Some stops have a telephone near the pump and you can use ir to find out the procedure for getting fuel.
These past few years tho, I have been finding some truck stops have upgraded their pumps so that you can use your credit card just as you do at the car pumps.
It is also very imperative that after you fill up you move your rig forward out of the pump area to let a truck behind you get in to refuel. It is not like at the car pumps where some park their car at the pump after fueling while they go do your errands inside the store.
Lots of stations have car diesel pumps in the car islands. These are usually the pumps on the end. I use them a lot. You can tell which are the diesel pumps as they usually have two hoses attacked to the pump. You usually find that the diesel hoses have a green handle. However, I have found some to be colored yellow.
Practically all the gas stations that have diesel fuel advertise their price in a green color.
Just use your head and be polite and you will get along just fine.