Hello and welcome, Nadine.
I don't know that anyone has converted the black tank to hold grey water, but I think it could be done. You would have to decide on how it would be connected to the grey water output from the trailer. Do you put a "Y" connection into the feed to the grey tank so that it would also empty into the black tank or do you simply add a pipe from the grey tank to the black one? I think the second approach would be better because then each tank would reach "full" at the same time, whereas the first method could get one tank full before the other.
However, clearances under the trailer are somewhat "tight" and adding a pipe to a tank wall is not particularly easy. Maybe you could splice a "jumper" from the output of the grey tank to the output of the black tank?
I believe other owners have replaced the flush toilet with another kind, such as a cassette toilet or perhaps one that composts. Do a search through the forum. I've had some less than happy experience with a composting toilet that was installed in a Nature Conservancy Preserve building that Janet and I did volunteer work in. I personally don't like them, but you may have had better results.
- Jack