Sounds reasonable.

I think you have it right, assuming you're not connecting the two terminals together :)

Getting the right polarity on the diode may present a small challenge.

You shouldn't need a charge controller - the truck's alternator handles that, limiting the supplied voltage to 14V.

You might want to install a diode to prevent the trailer battery back-feeding the truck.

You need a 12V adapter that can supply at least 0.3 A. If you have a 12 VDC adapter that can supply more current than that, it will work fine. If your adapter can only handle 0.2 A, you'll have trouble.

Yeah, I guess I'm a bit spoiled; my sky is reasonably dark. I'm in a fairly sparsely populated area a bit north of Toronto.

Best night vision is just a bit off the center of your view. Like a couple of thumb widths at arm's length.

I live in an area of moderately dark skies, and can just make out the Andromeda Galaxy by eye - and only if I have it just off the center of my vision.

I use a couple of stars from the Andromeda constellation to find it, though Cassiopeia helps too.

Maybe this will drive the price of dried mango down to something a bit more affordable than $10/lb.