If you look at the fourth picture down you will see that one end has a PVC pipe reducer and around that is closed cell neoprene insulation that I found at the place where I obtained the reducer. The inside RV vent has a slightly small ID than the deck plate and the neoprene means a water/air tight fit.
All I have to do to switch to the Diesel heater is change to the 3" sewer pipe adapter.
There is a lot of flexibility in the hose and I can see how you could have the AC under the trailer (ours will be in bad weather). One of my concerns was restricting flow of air to the point where the unit ices up and the plan is that if this happens I will put a 4" bilge blower in the air return line.
The deck plates are not expensive, I am looking at my spare and it was $7, the inside vent can direct the air with a turntable louver.