I don't know if I'm missing something or if this has been addressed before, but me and a friend just started co-op in hard mode and none of us can refuel a fuel carrier trailer at the gas station.
In single player, it works fine, but in co-op it doesn't. When you click refuel, it removes the amount of money from your account, but the trailer continues to be empty. We tried both to host and also both host and client refueling, with both host's truck and client's truck, same issue. Also tried restarting server.
The workaround was to close the server, switch to single player, refuel trailer, re-open co-op.
Anybody else having this issue? It's quite a serious one.