vehicle often runs out of fuel during the mission

The vehicle's navigation system doesn't consider whether the vehicle has enough fuel to sustain its journey. Therefore, if the assigned task is long, the vehicle often runs out of fuel during the mission.

My suggestion is to let the navigation system consider the vehicle's fuel level. If it's insufficient to complete the task, add a gas station as a link in the route, or consider making gas stations capable of refueling all vehicles within their range. Higher-level gas stations could have larger refueling ranges and faster refueling speeds. This way, all vehicles passing by a gas station would automatically refuel, making a widespread network of gas stations much more effective.

I'll note that some truck routes crossing the entire map are so long that a fully-fueled truck will run out before it finishes; there would need to be some kind of mitigation system to acknowledge when these trips are impossible.