"BIRDDOG-921".....There are two camps when it comes to cooling. Some like to run their roof A/C's because many have their families and pets in the back. Others, just the two of them, don't need to cool the back, and don't want to run A/C.
On our previous coach, the dash air just didn't cut it past 1:00pm on hot days. We did the adjustable shower curtain rod with a clear plastic shower curtain. This extended our cooling a little longer, but we often turned on our roof A/C about a half hour before arrival at our campground to get a jump on cooling the coach.
With that said, I doubt there would be a big market for a curtain that many only use a couple of months a year and can buy a shower curtain for next to nothing.
I think there would be a bigger market for a curtain like you're discussing for Class C motor homes to divide the front cab from the coach. The over cab bed makes for a perfect spot to hang a curtain. When we had our class, we always hung an old army blanket down from the bed to block the cold and heat.
Don & Mary
2019 Newmar Dutch Star 4018 - All Electric
2019 Ford Raptor Crew Cab