1st, there is nothing in a black or grey water tank that would be safe to dump on your driveway. I suppose short of hauling it to a dump station you could catch it carefully in a pail and dump it in a toilet. RUBBER UP first, eye protection and all.....
2nd, google is your friend, if you cannot find a user manual for it, whatever it is does not exist.
3rd, look at the tanks, no wires coming of the side of them then no gages. If there are wires there ought to be gages.
Good luck, enjoy the experience.