Can Grey tank be used as a Black tank?

Just got 2720 SL this week. Noticed no black tank..

Question. If the grey tank valve is left open, would the toilet's waste overflow into the grey tank? Can grey tank be used as a black water holding tank? Obviously using it only for #1.... Has anyone ever done this??? How much does the Thetford toilet hold before you need to empty. Summer campground does not have sewer! Please advise...