I have a 2016 F350 diesel towing a 2016 Voltage 3970 which grosses 20000 pounds (GCW is 31000), but anyone who can install a diesel aux can do a gas.
Mine was about $150 installed, 60 g, gravity flow to fuel line just inside filler neck. I leave the valve open 24/7 so I run the aux tank down before main tank starts flowing. I have a cover on my bed so I have the tank which stays 2 inches down from sides of truck (it is a flip cover so it is open when the trailer is hooked up, closed when not hooked up).
I love it for the fact I can fuel up unhooked (or use a truck stop hooked up which is way expensive) and can travel 750 miles without fueling when towing (sensibly I start looking for diesel when the main tank gauge starts going down), or 1200 miles unhooked.
CAUTION: If you use this particular type of set up, be sure to fuel the main tank FIRST, then CLOSE the cap before fueling the aux tank or you WILL get a gas bath. (Friend of mine did that ahem??)
Ken and Sherrie Pesce