Hi and welcome to the forum.
I don't think the problem is unique to older coaches nor does the bay have to have been flooded.
We've been kicking around eastern Canada and New England for the past six weeks and have dealt with a good bit of rain. My bays don't leak but just from having to set up and break down in the rain the carpet-like covering inside them has gotten wet resulting in that musty smell.
At first I tried using a portable fan to dry it but with such high humidity that didn't work well at all. I ended up using a hair drier and then some of that stuff you sprinkle on carpet and vacuum up to absorb odor.
But, I too am going to look for these bags since that sounds like the best bet.
Best of luck.
Rick, Nancy, Peanut & Lola our Westie Dogs & Bailey the Sheltie.
2007 Itasca Ellipse 40FD