How about on the frame of the truck? A little battery box mount may be easy to fab if your frame allready has plenty of holes in it. Then just attach an Aluminum Summit battery box.
I'm trying to put an air compressor on my frame. But everyone puts them on the outside of the frame rails where road spray soaks it. I may have to build a box or cram it into the crowded area on the inside of the frame.
A simple battery deosn't really care it it gets dirty or wet like a more complex compressor, so that' really isn't a problem. I may mount my batteries outside my frame rail.
'08 Ford F250 XL
Half of a '94 Jeep YJ
'02 Honda CRF450r, '04 Ford Mach1