
My terrier was just over 15 when she died about a year ago, in the last few months she was pretty much as you describe and on very similar medicines. A food formulated for the elderly can be useful, amongst other differences they tend to have more vitamins as an older dog finds these harder to utilise.
My philosophy on care of the elderly at this stage, though, is feed them whatever they enjoy, exercise when they want it for how long they want it etc., nothing is going to turn their condition around so life should be all about extracting what pleasure can be and when it becomes a chore for them call it a day. My old soul was also troubled by arthritis which often put her off eating and looking back now I wish I had been more proactive in deciding to call it a day as I find modern vets are more reluctant to take on that decision for you than they used to but it is all easier said than done isn't it? :)