DrFrige,
We had a kitty, Sidney, that had the same problems you describe.
He used to be a large cat, but in his later years became VERY thin, and ate and drank constantly. We assumed it was renal failure, but since he was so old, we decided to let him pass on his own.
One day, I watched him walk away from the food bowl. He meowed once or twice, then fell over. He laid there on his side moaning, and I sat there petting him, thinking he was about to leave us. Suddenly, he got up! [:)]
About 2 months later, my mother saw him, and said he looked funny. She followed him as he walked into my parents' closet, where he curled up in the back corner. He quickly passed. It was like he knew it was time, and he wanted to go off and be alone.
The point of all this is that he NEVER seemed to be in any pain. He was still as loving as ever, it was just that he ate so much. I never really saw him complain, and he always seemed to be happy. You might think about letting him pass on his own.