sounds logical to me. I too will not use a black "toilet" tank. do remember though kitchen sink in houses in many county consider that water waste; black, cause of bacteria that grows from food particles. Which makes a little since. I think newer rv's are now sending kitchen sink to black tank. If you do a live in rv setup... you can recycle grey tank for not potable use like flushing. Maybe clothes washing and shower?? too. That will reduce overall water use of potable tank allowing for reduction of more storage. They even sell kits for this if you need. (UV bacteria light killer and filter) see
https://www.usi-rv.com/main.sc;jsessionid=17CE48D188BF63E0B52BAF935845D7A2.p3plqscsfapp003
Think there is a tank size calculator on that site too