1968 GMC Custom Camper

I have the 69 GMC with that set-up. On mine,if you grab the back of the switch handle and turn it towards the drivers side door,then the LEFT tank is connected. Center (straight back) is the main tank behind the seat,and of course right is right tank. Check those lines between the aux.tanks and the valve.On mine they were plastic and needed replacing. Also don't be surprised if when you try to use one of these ancient tanks,your fuel filter immediately plugs up.I put a large in-line filter on mine that looks like the muffler from a lawnmower and now I can use these tanks.But even so I have to occasionally remove the filter and either blow it out with air,or replace it.