How do you mount the Li cells flat, and where outside?
I put them in front of the camper. Built a box using a few pieces of cut down 2x4 (cut to the width of the cells, just over 1.5") for the sides of the box. The back of the box is just the front of the camper. I then put a plexiglas cover on the front of the box, so I can see what's going on inside (when the camper is off the truck). And of course some holes in the side of the box for wires. Caulked the 2x4s to the camper and use weatherstrip for the plexi cover. With the front rubber bumpers on the camper, there is sufficient space for the box to fit in the truck bed, but it would be worth checking this on your particular installation before proceeding. Mine is on a 2015 F150. So basically, a flat (mostly) sealed box on the front exterior of the camper with cells laying against the camper. Mine are mounted upright, but one could easily mount in a different orientation.
I also have a charge controller (Redarc) mounted on the right side of the camper near the front, just forward of the access hole. It is high enough that it doesn't interfere with putting the camper on and off of the truck (i.e., it doesn't hit the wheel well). My power from the truck comes in through a port just forward of the wheel well. Worth noting is that I have exterior mounts (Torklift) on my camper and do not use the standard bed mounts, so there is no interference between mounting hardware and the electrical wiring in the front right part of the truck bed.
I keep thinking I should post some pics on here but haven't got around to it.
Yes, pics required pls.