Under trailer mounted aux fuel tank?

Are you starting with a short bed truck, or the equivalent of a short bed truck without the bed?

 

I've never seen a trailer that would get that close to the back of the cab (on a long bed frame) that you couldn't fit an L shaped tank.

Do you have a photo of the actual trailer, or the exact dimensions?

 

He's probably talking about 60" cab to axle truck.
84" cab to axle he would be ok.
There are company's out there that will custom make aluminum tanks.
Just do a google search.
I looked into those for mine but bought a Kaufman mini 5 and didn't need them.

 

Sounds like you have capital to work with, have you considered stepping up to a class 8 rig and full size stepdeck? The hotshot setup would be a good backup if the big rig has to go in the shop for repirs

 

you don’t want to go down?

 

I think that Xray4 is politely telling us that it is not of our concern - which is understandable.