Under trailer mounted aux fuel tank?

Don't have the trailer yet, will be on order soon. Truck is a crew cab long bed dually f350. Here's a pic of how the trailer sits in a pickup with flatbed from dealer.

 

Need the clearance for pivot and angles on the flatbed, and I've measured the space under the bed and there simply isn't enough to fit any more than maybe 20 gallons.

That's why I'm thinking of putting it under the trailer. Plenty of space there between frame rails and cross braces. We're talking 3 ft wide by 12 in tall by as long as you want.

 

And this picture is a little bit deceiving because they have a spare tire up against the headache rack. I don't have the space for that. This truck in this image appears to be a longer wheelbase rig.

 

Thanks bud. My truck holds 48 now, and I can fit another 20 under it. To the tune of over $1,500 which isn't really worth it. I really looked into this. It really seems the best option is to mount it on or under the trailer. I don't plan on running any fuel lines or anything between the two or anything like that, just a tank kind of like a saddle mounted storage container but a tank instead, and then in a toolbox mounted next to it have the pump and long hose with nozzle that will reach the truck. Just posting here to see if anybody else has done it before and what they're experiences were really