We're in a similar position with our 12 year old cat who has CRF - he's got fluid retention too and has put on 300g in a week. The vet has given him two weeks - the little chap has got his appetite back and munches on his catfood and his medication too if we're lucky.

He still loves to be on our laps too but we have to help him up, ok we have another cat too but the decision will have to come from us at some stage or as you say, they go in their sleep.