Sorry to hear about your dogs
This must be a really tough time for you I can only begin to imagine...
I think the slings and support harnesses are more realistic to use when your dog still retains some of its mobility... if they can't get around at all on their own.. then using a sling to physically carry them around is probably not feasible especially if they're big dogs. The harnesses aren't as much for the last final stages of a dog's life, but more as an assistance for a dog that has difficulty doing certain activities or needs that extra bit of help to get around or when going for walks.. getting in and out of the car...
Hope you're coping the best you can