I was thinking the other day, once I get the flatbed put on my F250, Im going to have a pretty decent space in between the cab and the rear wheels. Then I got to thinking taht would be a great place to mount some fuel tanks. They would give me something shiny on the truck which is always good lol, and then I wouldnt have to worry about where im going to put the filler neck for the current tank. BUT im not sure if its a legal move or not. Thought maybe you guys would know.
I don't see why it wouldn't be legal, as long as they are properly mounted. Chevy has had their tanks mounted outside the frame for as long as I can remember (don't know about the brand new stuff though, haha).
i would advise making sure your tanks were OEM for something, i don't care what, as this tells you they are roadworthy. from there, mount them as well or better than they were in their OEM application, and you're good to go.
your big rigs all have external side mounted tanks, so that point can't be a legal problem.
but one point i would consider is how well protected they are from a side impact. maybe have a heavy piece of steel sitting outboard of the tank to protect it from the unexpected
Saddle tanks were outlawed on OEM light trucks by the DOT because of side impact issues. I don't know if retrofitting them is illegal or not...
Getting saddle tanks that can take a sender unit that works with your gauge can be an issue as well. If you don't really need the extra fuel capacity, I'd just put tool boxes there.
If you really want saddle tanks, take a look at the step tanks commonly mounted under the cab on medium duty trucks like F600+. That might make a nice step up to the bed.