Debbie,
I'm so sorry Tubby isn't feeling well. You sure are an excellent kitty meowie to be so attentive. :) Tubby and Ms. Peanut are so lucky.
I'm glad he ate a treat, used the sand box and is drinking. Those are good things.
When I worked at the vet, they sometimes prescribed anti-inflammatories for arthritic cats. I know in dogs they use Deramaxx or Rimadyl. I think the Cosequin is a natural medication, with ingredients like condriton/glucosamine. Maybe you can ask your vet for something stronger for when its cold like this. I know my vet uses Pred because cats tolerate it well.
The heating pad is a good idea. We have this thing for Livvy when it gets cold (she has arthritis too):
http://a1272.g.akamai.net/7/1272/112...599_23066P.jpg
You just pop it in the microwave for 5 minutes and it stays really warm for 12 hours. Maybe that could help Tubby? (We got it at PetSmart, but they sell it online at Drs. Foster-Smith.
Give my love to sweet Tubby. :)