Also depends on how much fresh water capacity you have...no need to have grey tank capacity more than fresh when boondocking - you won't have the water to fill it. So if your fresh water is only 20 -25 gals. or so, your grey at 17 gal. is about right...
FWIW, we added two
gray tanks (plus a black) in our '56 Safari instead of one larger one...they are linked together so both fill equally. We found that we could fit two smaller ones inside out frame better than one large one. So far, it's worked out great!