I see absolutely no reason why you could not do that, regardless of how you plumb it. It's your camper, you can do with it whatever you want. And personally, I wouldn't worry about the next owner because that may never happen.
Considering black and grey tanks are dumped into the same sewer pipe at dump stations, there's nothing magical about keeping the black and grey separate in an RV either (not really). I think the idea of multiple tanks developed because it afforded smaller tanks, but more of them, which resulted in the end with more tank capacity. And if using smaller tanks, it makes logical sense to use tanks dedicated to one bathroom or kitchen feature. After all, they all come together at one outlet (valve) on most campers don't they?
All my travel trailers and now the 5er had separate black and grey tanks and pipes that outlet to one valve. Using a Flush King, I'd back-flush the grey water into the black tank every time I dumped to help clean out the black tank better.
There's nothing magical about the tanks. The only set-back I see is, it may take a while to clean out the black tank from it's original usage, because 'stuff' does splash around in there and even clings to the roof of the tank, which never get's "flushed" out and eventually dries hard as rock. If installing a shower, make sure you connect the black tank from the shower floor with a P-trap to keep odors from coming up, and make sure the tank is vented to the roof.
Go for it!
