Here's a picture for reference:
I'm talking about the panel just in front of the rear wheel (with black plastic splash plate on it) that goes to a crease line (about height of top wheel well).
It looks like this panel is glued or welded (more likely glued, as the crease line looks like body putty) to the bed above it.
I've seen trucks with this panel removed, but has anybody made it openable? Modify it with a hinge so you can tuck a battery behind it. It looks like there's just enough room to put a U1 size battery back there............