We had a Transfer Flow tank in our pickups. I liked it because we could decide when we wanted to get fuel, not every 4 hours or so. Also, we travel a lot at night and could again pick our times and locations for fueling.
I don't think the tanks paid for themselves, for us its nice to have the extra capacity/range.
We now have a Sprinter/Thor and I have added an auxiliary tank. Total gallonage is 37 gallons and we work off of 32 gallons. With a ScanGauge II, we know our fuel used and available very accurately. With the ScanGuage II I would have no trouble using some of the 5 gallons in reserve.
For a lot of people it just will not matter.
Good Luck.
Rick