It's called "Trucks"
I saw that also and that is where I got the idea to do it.
I can't remember the make of the tank I got but it was from Summit. It came standard with the twist lock cap that was flush with the tank. I called the manufacturer and told him what we were doing and he offered a solution. The company is RCI, the part # we used was RCI-2150A.
The reason I wanted to mention RCI was I actually talked to the owner when I called them. He knew of the application and sent me the filler tube to replace the original at no charge!!!
Freebie!!!
It was his idea to use the boat deck mount piece.
A hell of a nice guy that actaully CARES!
He made some suggestions on how to mount the tank and also told me about some of the potential problems that might arise. He wanted to make real sure I didn't use the mounting brackets as the only support for the tank!. 15 gallons fo fuel is going to weight over 90 lbs. You need straps attached to the fram for the main support.