I do not know about adding an extra grey tank. However, consider adding a twist on waste valve. Go to http://www.rvupgradestore.com/index....OD&ProdID=2644
to see an example. If you coach is like mine, the grey and black tanks are on the same level. With this valve you could dump grey water into the black tank. In effect using the capacity of both tanks.
When your grey tank is full and the black is at 1/4, with this valve closed, open the grey then black tank valves. The tanks will even out. Close both tank valves. The process can be repeated until both tanks are full. When dumping, all three valves start closed. Open the twist on valve first. Then dump as normal.