Has your vet ruled out kidney disease? Myndi was showing some of the symptoms that Marlin has when I took her in for a check. I really thought it was diabetes, but her blood sugar couldn't have been more normal. She had been drinking gallons of water, and peeing constantly, but she was diagnosed with kidney disease. She's been on the K/D prescription food ever since, and is doing great on that.
She has weakness in her back legs - when she stands in one spot, all 4 legs quiver and the back legs are more balancing her than supporting her. Surprisingly she doesn't have a problem walking. We never have done long walks - just exercise around the yard - but she gets around okay. First thing in the morning the legs are really bad, and sometimes she will even fall over. I just stand her up and support her for a few seconds, and then she's good to go.
She also is getting a little confused. One day last week she wandered into the family room (which is off limits to pups), and got lost behind the sofa. My son heard her fussing and rescued her.
Of course the problem with the legs and the confusion, is just probably part of being old, as could be the same with Marlin. Myndi is pushing 16 and has been around over a year more than I expected, after how sick she was a year ago.
So for now, I'm just happy that she is content and happy and doing well on her special diet. It really made a world of difference. I hope you can get to the bottom of Marlin's problems and get him on the right track. It's just so darn hard to watch our furkids get old.........