The black tank flush is only for a single tank. You would have a second one for the rear tank. Unfortunately, these weren't often offered (maybe that's changed). The owner might have installed an aftermarket one- if so, look for a hose hookup back in that area.
As for which handle is which, if they aren't labeled- the easiest thing to do is to test it. Empty all of the tanks first. Then, fill a few gallons of water into one spot (say the sink) and then go pulling handles to see what comes out (a clear plastic elbow helps a lot).
If your layout is like mine (Sabre 36QBOK which is a bunkhouse), then that is possibly your kitchen sink.
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