Thanks, guys!
Landy, I like the idea of an access hatch. I know other guys here have told me if I put in a proper panel I will never need to access the terminals, but, well, I am not that trusting :).
I was thinking to put them on their sides for access, if no hatch. And if course to put 2 on each side of the rig, as far forward as practical. They'll be behind the front seats.
I was planning to put them on the outside of the frame, which means the frame will be between them and the exhaust system. Cooler, but more vulnerable in a crash. This is where standard/aftermarket smaller batteries go. What do you think?