I fixed up a V3S, I also found a gas can and filled it up 4 times. I can drive the truck as long as I want, but the second I turn the turn off, and exit - the vehicle runs out of gas.
All I have to do is exit the vehicle for it to be empty, I also could fill it more then 4 times with a jerry can. It still drives as long as I want, until I exit.
Exit vehicle, refill with 4 jerry cans - drive it - then exit.
Vehicle was not damaged, I drove it on one canister until it ran out of gas. I did store the jerry can inside of the truck; maybe that might be a clue.
Actually I've had this problem Numerous times, if you have bumped into Anything, and i do Mean Anything. It causes the Gas Tank to Leak. so at some point you've had to of bumped into a Bush, Fence, Tree, Etc. But even Driving you Continuously are losing fuel and will even run empty while driving, once the Server Restarts the Truck will be fully repaired and will no longer be leaking fuel, that is unless you bump something again.