Portable house question.
I’m building my 2nd campervan.   Moisture condensation is something we worry about since the van skin is steel.   Fortunately it doesn’t seem to be as bad as one might expect, but its not be be ignored.   The main defense is to minimize the humidity with proper ventilation.
I plan on a similar build to my last one
Floor: in order from bottom up.  vantred (PE foam), XPS (1/2 or 1″), plywood (1/2″), vinyl sheet.  Every layer will be sealed/caulked well to keep any water out.
Upper walls and ceiling:  Polyiso foam boards glued down and sealed around the edges with great stuff.

Lower wall:   This is where my question is.   The last time, & I’m leaning toward the same method this time, was to use Thinsulate (sound and thermal insulation).   The reason for using this is because it’s really easy to work with in odd shaped cavities and for the sound insulation benefits.    Last time I used a house wrap (probly tyvek or HomeGuard)  air barrier over the top to seal it in the best I could.   (It would have been interesting to tear the old one apart to see how it did, but I didn’t, and it’s gone now)  The idea is to keep the bulk airflow out, but allow any moisture that did get in there to evaporate.
Does that seem reasonable?   
Should I use Tyvek or HomeGuard, or is there something cheaper or in smaller rolls that would work.    I see some building paper much cheaper, but it’s asphalt saturated (not good inside).

thx, brad