Here's one....$35.50 each plus shipping.

http://www.batteriesasap.com/48.html

Also an 18AH size is available....every so slightly thinner.

http://www.batteriesasap.com/sla1712.html

http://www.batteriesasap.com/659.html

Have you got any better brands...soources....Mark?

I'm excited about this possibility...looks like we can potentally get
quite a few days worth of camping AH for an electric fridge/ FA furnace,
etc, withuot an expensive solar rig to supplement.

"Hinged trays under the floor" ?? Hmmmm. Like to see how these would be
designed/made.

Also like to know how many will fit in the engine bay box,

 ......because I've been coveting that same space between the frame rail
and the outer sheet metal for a custom tool and "rough" accessories
box ....possibly accessed from a lid cut and made into the floor...would
be easy access near the sliding door ...and the lid hidden under a piece
of carpet.

You're the lead on this Mark.  I'm going to order four of the batteries
right away for trial fitment purposes.

Warren C