What I do for the vent is to use the AC as a "slider" setup, so it pokes out the front of the opening it normally travels in (the stowed position, aft). The box it sits in will have a 1/4" slope per foot at the bottom to the outside (toward the front, for condensate & weather drainage) and a removable face panel of aluminum just above the tongue box, facing the front. To use the AC, it will be necessary to slide the front panel out of it's track first, then the unit slides forward until the face frame around the unit (near the control panel at the "back" side or inside on the AC) seals against the face frame of the cabinet box it sits in. This provides an "airtight" seal for bugs & weather. The slope of the bottom shelf & the drain lets water run out & onto the tongue box.
The inner sides, top & bottom of the box will be laminated with thin aluminum to seal it from weather & rot. The entire AC unit sits about 6" out at the front once it's pushed into place (the back end of the AC unit itself), this allows for air flow to provide dumping of the heat to the outside while running. Unfortunately, it also sits about 6" to the inside when travelling. I'm using simple pins on both sides to lock it in place for travelling, so it can't move front to back. Once it's slid to the outside, gravity should hold it in place. With the tongue box being 22" tall, the AC starts at about 23" above the deck on its shelf, so airflow is directed over our heads for sleeping & naps. I thought about having it in the tongue box at floor level, but I didn't like that type of setup. This was the best of some bad choices due to size & everything else I had going.