I've already looked through the US Holocaust Museum, and was able to obtain relevant documentation about some of my ancestors up until 1941. Is there any chance of being able to find documentation between 1941 and 1945, or has most of that been destroyed? If it helps, they were in the Krakow Ghetto up until 1941 and were (almost certainly) transferred to concentration camps after. I would appreciate if anyone could point me in the right direction.
Your best bet at finding individuals is through the International Tracing Service, which USHMM has access to. The Central Names Index contains about 50 million items relating to some 17,500,000 people persecuted by the Nazis. If your family's information is available, it's likely included in ITS. USHMM is the only location in the Western Hemisphere that has access to ITS.
Did you speak to the archivists there when conducting your research? ITS is a difficult system to learn, and it's unlikely that you figured it out (at least fully) on your own without any help.
If that doesn't work, my next step would be to start looking at scholarly books on the Krakow ghetto and its liquidation, and then check the notes to see which archives they went to. But this would almost certainly require a trip to Europe.
Also, you might consider writing an email to Yad Vashem. They will have resources unavailable anywhere else save, perhaps, USHMM.
Good Luck.