Cheapest way to charge two batteries ?

Brad, forgetting to flip the switch is something you so might easily do so here is a suggestion I picked up from when I was a lighthouse keeper.
We used to have power provided for the lights with a back up generator in case of failure. The head keeper would test the generator occasionally and so that he didn't forget to flip back to normal power he used a yellow circle of linoleum about 10" across which he dropped on the ground at the place he would be leaving to go back to his quarters. If he forgot to flip the switch he would see the yellow patch. He would then pick up the patch, flip off the generator and leave the patch in its place.
Anything bright like that would do the trick. Linoleum, a flat circle of tin painted brightly and heavy enough that it would't blow away with a puff of wind. Perhaps the place to keep the reminder would be next to the battery. When it is on you leave the patch perhaps next to the driving seat or your cooker, on your swag so that you see it or the last place before you are likely to be just before forgetting.
I use a USB port for files which I take backwards and forward between my office computer and home laptop. So that I don't forget to take the port home I leave my car keys on top of the computer block where the USB is plugged in. That way I can't leave without the keys and there is the USB.

And while on the tip theme. Whenever you camp and pack up to move on always get ready to go and then walk around your vehicle and camp site 3 times to make sure everything is secure and you haven't left the frying pan next to the tap or a shovel under the car.
Have a great trip.

this is great advice, it will work but if you forget you could be in flatt batt land.