Reddit mentions of Femitu High Current Toggle Switch DPDT Center Off Momentary Both Sides 20 Amp

It's safest to have the ground of your 2x6 battery "bank" permanently connected to the ground post on your main battery, and have the positive side switched. 2 gauge wire is major overkill. 8ga is plenty. But what you're describing sounds a lot like what I've seen 100 times in bug-out and camper type trucks; A separate 12v source that's charged by the truck when running but disconnected when off.

Now, the question is how to control the power slider motor from outside. I assume you're comfortable getting access to the slider motor under the trim panel. Once at the motor you'll see it has 2 wires running to it. You'll need to tap each of those wires and run your new lines down and outside the cab, mating up with your 2ga charging lines and bring them back up and into the bed area. From there you need that DPDT Center-Off switch. One like this. The reason you need that specific switch is because with the switch in the center everything you're adding is disconnected from the main system and you can still control the window from the driving position. You also want to add a 10A circuit breaker so that if you accidentally leave the switch clicked on too long the breaker will protect the motor. With the switch getting (protected) 12V on one pole and ground on the other, you can wire it so with switch in position 1 you Power wire A and Ground wire B. In position 2 you Ground wire A and Power wire B. This give you control in both directions, while maintaining full control within the cab.