At my old job we ran 6 or 7 crews daily, what we ended up doing was, we bought a lot of extra 5-gallon gas cans, and someone would fill the empties during the day and when the crews got back they would switch their empties for full cans. Also they would top off all the mowers at night.
Each crew would fill their own trucks when on the road, preferably NOT first thing in the morning when it was a zoo at the pumps. Typically they would pass many gas stations during the week, they were told to fill up when it was convenient.
It didn't always run smoothly, but that seemed to work the best over anything else we had tried.