well, black tank is for the toilet.
gray tanks are for kitchen sink and usually bathroom sink.
gray tanks fill much faster than black tanks.
you should have two gate valves, one for black and one for gray.
some RVs have two gray tanks, one for bathroom sink and one for kitchen sinks.
suggest you do a search here on both. or Google them or YouTube them.
and Zoe the Wonder Dog(R.I.P.)
'07 Roo 23SS pushing an '07 Chevy Avalanche
Equalizer WDH and Prodigy BC