May I ask what you are feeding for dry dog food? My boxer went through something similar to this (kidney function was WAY down). I had him on homemade meat and rice (liver, burger, chicken, or turkey and brown rice) for about a month, then I started to introduce him to Solid Gold's Hun-den-Flocken dry food. It contains no grains other than brown rice-no corn, no wheat, etc. that is very hard on a dogs system, esp. an older dog. Since then, I have switched him (and all of my dogs) to Canidae All Life Stages Lamb and Rice formula. It's comparable to the Solid Gold, but a little more cost effective, and the kibble is smaller so it is easier for my little dogs to digest. Canidae also makes a very good Senior formula food, but it is easier for me to just keep one food in the house and supplement my boxer with glucosimine for his joints.
My heart goes out to you and your situation! My boxer's kidney problems were solved by doing the above: he has been healthy as far as his system goes for 2 solid years now. However, his knees and hips are pretty much shot now. He doesn't appear to be in any pain at this time, and he can get in and out of the house, on and off of the couch, and he can even run on occassion. I am not sure, however, how long his poor old joints are going to take the Wisconsin northwoods winter weather. He'll let me know when it's time, and I, too, will be there to escort him across the bridge with dignity and love.