Elkmont flush toilet

The Elkmont we've purchased has the flush toilet with the 15 gal black water tank, not the re-circulating toilet (Thetford?)

Does anybody know how many gallons it takes to flush this toilet? (or how many flushes per 15 gal tank).

In case that's not good enough, I'm sure I read somewhere here about the possibility of replacing the flush toilet with the re-circulating model and keeping the black water tank for additional storage. But i can't find that anywhere. Can someone point me to that thread? Has anyone actually done this?

Thanks ...