I recently installed an 85 gal. fuel tank from a semi in my truck.Plumbed it into the filler pipe with a manual valve. It worksd like a dream as it flows 1 gallon per minute so when the main tank got down to almost empty I stopped, turned the valve andf drove for another 25 minutes to refill the tank, then stopped again and shut off the valve. Took approx 30 seconds to stop, turn valve and return to driving. If I was going to use this a lot I would probably install an electric valved,
Prior to my 1500 mile trip (first leg of a return trip) I sat down with gas buddy and determined the best place i,e, chepest to refill along the way. I was able to make the whole trip on one fill-up at the cheapest place in Canada for fuel vs stopping 4 or 5 times where it was more expensiver and inconvenient.
The tank is round and takes up a little more space than a rectangular one but it is easily removablde (approx. 15-20 minutes) for when I don't need it.
Installation was easy, operation is simple and I have piad for the whole thing in the first trip.