Gary,
(I realize you are interested in Ford diesel...
but will share my experience with Gas Transfer Flow).
I have a 35 gal. Transfer Flow mid-ship replacement tank on
my 2003 2500HD. Truthfully, I was never aware of any legality
issues with "gas" tanks. I had it installed locally just about 10 years
ago...they never mentioned it. I have not had one single
issue with it... or with any GM warranty work.
I have no regrets. I do recall it was "pricey" at the time...
but, like you, we desert dwellers can often find it long distances between
stations. I live in Chandler. Have since trained myself to look for a station
when gauge reads 1/4.... the 8.1 L never met a gas station it did not love.
My un-hitched mileage is not impressive...my towing MPG even
less so.... both reasons for the tank purchase.
Good luck in your hunt.
Charles