Another thing to consider is the underbelly heating. My trailer is like that also. I had the underbelly off and was working in the bath and suddenly the cat hopped up in that plumbing chase to say hello a while back.
If you have a heater hose from the furnace that goes into the underbelly, it has to have somewhere to flow back upwards into the trailer to circulate. Otherwise there is no real heat flow. I think that is why some of those openings are like that into cabinets, etc... The leaking around doors, etc will let the air flow some (and its cheaper construction). No facts to back this up, just a theory so anyone else chime in here.
Not to hijack the thread, but those photos also are great at illustrating exactly how little to no support is actually under those shower pans. That is what I was looking at in mine. I'm a little on the "corn fed side" and I'm thinking of pulling my base and laminating in some extra fiberglass and epoxy and 3/4" plywood on the bottom with more supports. My base and sidewalls are 2 pieces unlike some. Back to the plumbing thread....
Just the 2 of us in a...
2015 Freedom Express 246RKS
2002 GMC Sierra 1500