Nice looking pups!
I was working out in my rig yesterday removing the aft dinette seat plywood in my 251.
This access cover is screwed down and has seat belts that go through the plywood.
Because I was running the furnace the first thing I noticed was how warm the seat custions were, and when I lifted the seat plywood I saw why!
Trail-lite had run another 4" flexible duct under the seat that was not hooked up to any register, it's just heating the dinette seat box.
I also noticed that Iv'e seen interior pictures of 2008 T/L 251's and they now have the additional register in the isle.
So if any 251 owners that don't have the additional register in the isle want to add a heat duct, your only a 4" hole saw and a $4.00 plastic register away.;)