Here's the backpack/bag section of the .org if you haven't seen it yet: http://www.he-man.org/merchandising/section.php?id=69
I've never seen the bag you described but I can definitely relate to your post here. I had this duffle bag that I carried all my MOTU/PoP figures around in (even on the carry-on in planes to Florida and back every year):
Your bag is probably made by the same or similar company if it looks like a similar design. I've seen MOTU/POP bags on ebay a few times over the years...but they are definitely not a common thing.
I even picked up this Evil-Lyn tote off of ebay several years ago to carry my video game controllers, liquor bottles, whatever the case may be to my friends houses
Post a pic when you can! I'm interested in seeing it, especially since I'm not finding it online anywhere either. It might be in one of the Sears Wishbooks on wishbookweb.com. I'll check those out when I can since it can be narrowed down to 1985 to 1987.
Edit: Is this it? #26 next to the She-Ra sleeping bag?