I think that if you can figure out self driving trucks, you can figure out keeping it fueled as well. As a quick and easy solution I would imagine large enough fuel tanks on the truck itself, and a couple of gas stations with people fueling the trucks at places. One for every large industrial place would already make it very widespread. Another solution would be mechanical arms attaching to the truck.
You can always pay a human to do what a computer can't, until it can.
Some truck stop fuel islands have a lane where a human will come out and check/fill your tires. You pay for the service, and it's not much.
Most humans, or the occupations, are cheap, as you can see when you think about all those humans paid to stand on street corners and wave signs. Cheaper than paying to plant the sign on property, and a cheap enough workaround when you can't plant the sign at any price.
Worst case, it signals a human fueler in advance of it's anticipated need to stop for gas, and the fueler meets it and handles fueling.
Over time, though, building a network of compatible automated fueling stations to support the trucks makes sense, and you can use data from the trucks for siting decisions.
