I find an aux tank a must have. Don't need to go overboard like me with a 91 gallon tank, giving me a total capacity of 135 gallons, but it sure is a lot nicer only filling once every 2-3 days vs 2-3 times a day... Just bites breaking the wallet out to pay for $500+ worth of fuel every time I fill.
IIRC both of my Ford superduties only had 29 gallon tanks, simply doubling that would make a long trip a lot easier.
I use an RDS tank from Northern Tool and use the gravity feed hookup kit they have that ties the aux into the fuel tank filler neck and uses a check valve. Works great and not too pricy like the transfer flow systems.
FWI, using a gravity feed system with the fill valve always open WILL set a check engine light code for the fuel level sensor in a newer Ford ( It did on my 2006 F350). Best to run off the main tank and top it off with the aux tank as needed.... or carry a code scanner to clear the code.
I'll try not to break your heart and tell you my lie-o-meter tells me 24 - 26 mpg empty and 13 - 15 towing.... It's usually 1.5 - 2 mpg high from my hand calculated numbers