They had a gas pump on the farm with an in ground tank for filling the tractors that he used to fill his truck. I'm guessing when they got the gas/water out of the tank they put a gallon or two of fresh gas back in and drove to the farm to fill up. Trucks back in the 50's were easier to work on so I doubt there were any problems. Plus my Granddad could do anything mechanical. I have several tools my Granddad made for the job needed if he didn't have the right tool.
Funny story about my Granddad. There was a large shop attached to the hanger at the landing strip his brother used when he flew to the ranch. He'd be in the shop working on a tractor, bulldozer or whatever. Since it was cold outside, the hired workers would come into the shop and go pee in the corner. As the temp warmed up it got to stinking and my Granddad dug it out and put lime or something back in the hole. When it got cold again, the workers would come back in and pee in the corner again. He told them not to do that anymore. But they did. So he put a metal plate under the dirt in the corner and ran wires from it to an old crank generator on his work bench. The next time one of the workers came inside, he waited until he heard him going good and cranked the handle on the generator. He said the man screamed and yelled and ran out of the shop. He broke them of the habit.