Thanks for the ideas, Chuck. I'll put up a warning flag
I had some of the heavy, plastic bags for Adopt-A-Highway pickups that followed me home. They're similar/identical to the trash compactor bags. I've been using them to protect my sleeping bag from wetness.
(1) Stuff sleeping bag into its original small, stuff sack. This compacts and protects the sleeping bag from physical damage from tearing, ripping, etc.
(2) Put that bag into a HEAVY plastic bag - trash compactor or similar, thick-walled bag. (I also keep my extra AAA & AA batteries in here, taped together with all positive ends aligned, in groups of 3 or 4, as needed. Then, wrapped well in plastic wrap (Saran Wrap) to prevent opposite poles touching.) Squish all of the air out of the bag, tightly twist the neck full length, then bend that twisted neck over double into a "goose neck". This proyects the packed sleeping bag from water.
(3) Now, stuff that tightly wound, goose necked, plastic bag into another nylon stuff sack. This protects the plastic bag from physical damage of tesring, ripping, etc.
(4) When you teturn home remove the sleeping bag and hang it up to dry and air out. Store it NOT in the stuff bag, as that will ruin the loft of the filler - which SHOULD be down. A large, mesh bag is good for storage. I go to a multi-market store (department store/grocery store combo, and get the largest, mesh bag I can. They might be in camping, or might be in home goods nesr Laundry. They also make nice bags for dirty clothes.
Some campers think that's too much work - until they find their sleeping bag all wet!