Assigned truck to fuel station should stay at station

Many times I find when my excavators are out of fuel its because the truck assigned to them is half way across the map. I dont know what they are doing there as they dont need fuel (its piped in to station). If you assign a truck to a station they shouldnt leave their post.

I observed the assigned trucks going to fuel other excavators that the fuel station was not assigned to. This would imply to me that there is no way to keep the trucks at the station and only service the tower the station is assigned to. Should be an option that would keep the truck from servicing other excavators.

I have not tried it yet, but didn't we get "zones" to help with this problem?
I'd suggest looking it up on the WIKI but the stupid thing is so out of date it does not have anything about the new update.
Shame that a game this complex does not have any sort of manual or reliable source of information about game mechanics.