Gray tank inside van under shower pan?

Ok, forgive me if this is an absurd idea but I was thinking about putting the gray tank inside the van under the shower pan. I see some 24x36x7 gray/black tanks that are just 7" tall. So it would be a step up into the shower but that seems ok. I’m not worried about the loss in shower height. I would frame in the tank so there isn’t any weight on it while showering.

Has anyone done this, or knows it would be a bad idea?