Please take no offense but to me, what you're offering is a solution looking for a problem, one that I don't have.
A couple seconds to go pull the black tank handle, take a little walk, come back, close the valve and pull the handle to open my gray tank to flush the hose every 4-5 days (30 gal black tank, 3 people in the family) is not a problem I need a solution to. Especially at a cost that I could easily replace all our camping chairs (4) and tables (4).
Maybe if you gave a little more detail you might get the "Thats a cool solution" factor going but to me, $200 to solve a problem thats not a problem is money better not spent.
Good Luck with your idea.