Buster.....
They really are not "shower curtains" per se and the fabric is not porous so air cannot bleed through it. My wife bought the fabric that we used from the department in the store that sold shower curtains and bathroom supplies. When you pull the two opposites together, they overlap so none of the cool air get past them.
Our coach is a "class A". It's a 2008 Monaco Dynasty 42 foot motorhome.
The whole operation is not that difficult and certainly worth the effort and it serves the purpose. You'll wonder why you have not done it before.