Hit The Gas (Mary May)
After returning the Widowmaker to Fall's End, you'll get a call from Mary May about low gas reserves in the town, which triggers this mission.
This mission is eminently skippable- it's random, mildly irritating, and it'll autocomplete when you liberate Holland Valley anyway. If you still want to go for it, then get started by tracking down one of the fuel trucks roaming around the Valley. Watch out for roadblocks, checkpoints, aerial spotters, and random baddies as you go. Try to avoid fights, as they tend to escalate quickly.
An ATV is good here- nimble enough to avoid trouble, and when you do come across a tanker, you can just pull alongside and pop the driver with your sidearm.
Wait for the truck to roll to a stop, then hop in the cab and make tracks for Fall's End.
You'll have to do this twice to complete the mission.