My impression is the thing I'm pointing at in the picture is a special riser for the TrailManor and similar trailers with not much headroom. Other trailers seem to mount the AC units more directly but if we did that, it would bonk the taller of us in the head as it would drop down too far (although the air flow would probably be better).
I also noticed the gasket it has (the one between the roof and the riser) seems manufactured. At least, not applied by hand from a tube. But I do wonder why the front of this riser between the lower riser and the roof has some additional caulk gunked on. I'm still debating if I want to dig into it. I halfway suspect it might have been an attempt to fix a leak that was more likely coming from the seam and not this area so the caulk is a red herring but...
