We don't even have a grey 'only' tank.... we have ONE holding tank for grey AND black.... 189 gallons. The fresh water is also 189 gallons. It just made total sense to me when we were picking out a converter that the one-tank set up was a good thing. Not all converters are the same, very different actually where they put stuff (generators, generator radiators, etc.).
The RV world often says to make sure and put water in the black tank, that a black tank cannot have a enough water, etc. etc. So, that's our set up.... grey and black together.
MM.