Keystone Travel trailer faulty design

Looks to me like the slide situation is normal, with th exception that instead or running the carpet out a little further and tapering down to the floor, they stopped it at the end of the slide. It depends on which slide mechanizum they're using but most I've been in are raised at the edge. Slides are nothing more than 3/4-1" of plywood covered with carpet. When they extend they need to stop short of the main floor to seal right. They basically are sliding in and out over the interior floor. Looking around the net at your trailer I've seen what you have and also the same trailer with the typical 5-6" piece of carpet tapering down to the interior floor. Looks like a change midway thru production. Once again that happens often with all brands. Just a though but you could always have a carpet store run some new carpet out further to make the edge less abrupt.

As far as the floor vent working when the sofa is out in hid-a-bed mode. All it will do is make the air circulate out from under the bed. The bed certainly won't trap the air. You could always replace the floor vent with one that opens and closes. Just close it when the bed is over it. If nothing else you'll have a nice heated mattress.

The issue with the single cargo door is IMO a major dumb a$$ move on Keystones part. I would definitely try and pressure keystone on that.