I looked at the TransferFlow when I bought my truck: 2001 Dodge dually. I just didn't want to spend that much for what I considered a convenience.
However, now that I have had an auxiliary tank and used it for a couple years, it is much more than a convenience to me. I still am too cheap to spring for a TransferFlow though. :) The main thing I love about it is the ability to only fill up once a day. Which for us is usually in the evening after we've unhooked. That way we don't have to worry about getting into and out of a station. It really isn't a big deal on the interstates with the big truck stops, but when you get off the beaten track a bit, sometimes that 40' toyhauler can hinder easy access to fuel. :)
My wife bought a white 35 gallon tank from Tractor Supply, and a new unopened pump/filler hose w/nozzle off Craigslist for me for Christmas a couple years ago.
I installed it myself which consisted of putting it in the bed and drilling some mounting holes. I have it set up like SilverCorvette and just have clamps to attach to the battery when the main tank gets low. Total invested is about $500.
I love it and would do it again in a heartbeat.