If you want to keep the AC outside on the ground, I think the world is open to you for how to do the vent since it's just a matter of getting a vent cover that you like for the inside (that opens and closes) and cut a hole that big and get one of those thin aluminum duct adapters with a hose. Maybe come in just behind the galley wall under the tear, and run a permanent duct to a couple of small vents blowing toward you just above your feet. A concern that comes to mind would be that the wheels might be right in the way though.
Here's a pic I just drew of what I an thinking of, sorry no CAD here just old fashioned pencil and paper...
I would consider the possibility of installing it inside the tear under the galley counter with the face sticking through the galley wall into the cabin, but that's me, I just want to hit the road in a moment and then pull over where ever has power, climb in and crank up the AC, remember anytime you are going to hook up the AC it is probably going to be hot outside and lugging the AC out of the back of the truck or wherever is going to be a sweaty hassle. Not to mention if you need to make an adjustment, or turn it off at three in the morning, you will have to get up and get out, and go outside, after which time, you will probably get hot and need to leave it on longer lol... OK no more harping, here's my pic:
Note there is a hole in the bottom or the shelf, the airflow would be fine without it, but with that hole there the air flows straight up through it. the vent to the side of it is an additional air intake that fools the unit into
One thing I made sure of was to not cut ANY holes in the walls or floor until I was absolutely ready to do that part of the build. I am soo glad I waited because my AC plan changed numerous time and if I had cut holes before the walls went up, I might be in a world of hurt right now, or at least be stuck in an old plan when I had thought of a new one a month later. It's sort of a tightrope walk of planning ahead and not doing more than your current job at hand.