What if you top off the black tank while showering?? That water would be in direct contact with poo. That cant be safe. Nothing like doo doo feet to start your day. Who designs this junk and what kind of clowns are sitting in that board room saying "yep we think this is a great idea". Maybe if a little Japanese competition was introduced to the market we'd have some better RV's and trailers similar to how American car manufacturers were forced to quit making junk like they did in the 70 and 80s.
It may be easier for us to tell you what brands do not do that. My keystone passport shower drains to the grey. Also the restroom sink does too.