So what exactly was in the bathtub at the end?

Mrs Lipton an old lady who owned the house who Jame Gumb and Frederica Bimmel used to do alterations for.

Starling questions an old friend of Fredericas about any bfs. The friend said tailoring was Frederica's life. After further questions, the friend claims Frederica done a lot of tailoring for Mrs Lipton.

Buffalo Bill (Jame Gumb) has killed her and taken her house. Which is why the fbi have the wrong address in Chicago as the house he is in is not registered to him.

I believe it might be this old lady Mrs Lippman that employed Frederica (the first victim) for sewing jobs. The Buffalo Bill killer mention Lippman's son as well.
But the movie being still fresh in my mind, it looked like old lady hair.
Maybe some hardcore fans of the film knows the exact answer, but I assume the killer didn't bother disposing of a body of a person he's not having fetishes for.