Not sure where you'll find room - but battery mounting to frame rails is pretty common for hot rods. You can just buy frame mounts from a variety of vendors.
I think the Sportsmobile vans had them mounted down on the frame rail OEM.
That said - I've never needed a second battery. My fridge will run off my regular battery for around 4 days without a recharge. With a 100w solar panel - it's really never an issue. And I guess I haven't used my winch enough for long enough in a row to kill the battery like I do on my FJ40 because I just don't use the 4Runner in a way that I'd do 5 or 10 back to back full length winch pulls.
I'd suggest just buying a lithium jumper pack in case you do take the car battery down too low and use the regular battery. It's worked pretty good for me for 6 years.