not permanent
I have a gravity feed valve that will feed into my stock tank if I chose.. I control it with a ball valve to control the flow, and feed that into my filler neck of the truck, very easy to undo as well if you chose to not use it as an auxiliary tank.. when I would use it, I would leave it barely open and when I would see my factory tank was full, id pull over and close the valve...
now I did remove it, not for being hassled, but because some joker where I work thought it would be funny to open my valve and after a 16 hour shift, well I had no diesel in the auxiliary tank, it was all over the ground by my pickup...
I removed the valve, put a plug, and removed the rubber line feeding into the filler neck.. all of a 10 minute job... and I have the parts should I chose to run it again...
is it legal? not sure... IIRC, I think anything over 140 gallons in a 1 ton or smaller needs placards?
so I never fill my tank all the way, especially when my factory is full... I have a 110 gallon right now, and got a 90 gallon I got as a gift that I will replace the 110 with..
that tank is going to a friend of mine, and with the 90+35, ill never be over the legal limit for placards...
just food for thought...
anyways far as your question goes... I don't know how transfer flow does theirs, but my dad and I have always done ours this way through the filler neck.. its not that hard really...