[Diesel-Gypsy]

Well, yes it does seem odd, but it is true, lots of air movement there.
In theory there should'nt actually be a draft, that would actually mean that the basement IS NOT air tight.
Well, on my 36TK3 that basement area was full of places that created drafts.
Those drafts don't seem to mean much when the OAT is in the mid 70's to mid 80's.
As it gets colder they really become a PITA.
I could tell you where all my sources of air infiltration were, but is is best if you just get in there and start finding them yourself.
Look first behind the sliding doors that conceal all the plumbing & wiring.
Look down where the sewer lines make there way out of the basement.
Also look around at the tin-box that separates the propane tanks from the basement area, the tank side should be as cold as the outside air because it is vented, but because it is not insulated it becomes a huge direct link to the cold outside temps.
The forward mounted battery box is also vented to the outside, and not insulated to the basement area.
The Landing gear legs pass through the basement, they are usually not well sealed to the outside.
Then the biggy, the entire basement area under the main floor is open to the cargo area of the basement as far as air movement.
The belly area around the spring support drop-downs, and many other areas under the trailer are not fully sealed, and air can easily make it's way into the underbelly of the trailer.
That is where many and or most of those drafts were in my rig.
There are more, but I think by now you may have the gist of it......

Rick