From what you are describing, I would agree that it's likely arthritis. Everything from her decreased appetite to the stumbling after she gets tired. The discomfort will cause a decrease in her appetite. I agree that shorter, more frequent walks build muscle better than longer, less frequent walks - and keeping the muscles strong helps to stabilize and cushion the joints. I would caution against allowing her to jump in and out of vehicles - that jars the joints, and for an elderly dog, the impact is too much.
Depending on the brand of glucosamine you are buying - you may not be getting what you are paying for. Supplements are not regulated like drugs, and many brands of supplements do not contain what they claim when tested by independent labs. And like Hazel said, it could be that you aren't giving the proper dosage. You could also talk to your vet about using an NSAID like Deramaxx for the days when she is bad or if she's been overly active and you know she'll be in pain.
Her scraping her back feet could be due to a loss of range of motion, which is common with arthritis. Or perhaps the walks are too long for her and once she gets tired she doesn't lift her feet well enough? Shorter walks would help with this too. We had gsd's when I was growing up, and once they got to be about 12 or so, they definitely slowed down quite a bit, although they could show the bursts of energy when the mood hit them.
For managing her condition, I would suggest continuing the g/c supplement (either changing brands or increasing the dosage as appropriate), keep her weight on the lean side, don't let her jump into or out of vehicles - or on and off furniture, take more frequent, shorter walks to build her muscle tone without undue stress (swimming is also an excellent exercise and has zero impact on the joints), talk to your vet about the use of a painkiller for when she needs it, and lastly - look into buying her a good orthopedic bed. Allowing her to lay on hard floors is only going to increase her stiffness.
Livin in a Newfie Drool Zone