I have the reserve tank in my 72' and when I switch the fuel cutoff to them the fuel gauge stays the same, should my factory gauge be reading the amount in my reserve? I know most new ones do but not sure anout the 72'. For now I just have to get out and knock on it to see where the fuel level is. Just curious, I want to drain the thing completely as I don't know how long the PO had that 12 gal. in there.