These things are called "bag seals". If you do a Search on that term, you will find lots of good info and discussion. Here is a particularly good link.
https://www.trailmanorowners.com/for...ad.php?t=15153
I seem to recall that you can get replacement fabric directly from the Trail Manor factory, but there is a fair amount of effort involved in forming/stuffing/stapling them. One of our members did a small repair using Flex Seal. You could do a Search on that as well.
Bill