I've been working on designing similar modifications. You might want to check out the following link.
RV Tanks
The tanks are all designed with a pitch towards the drain and have fittings for whatever configuration you desire. I'm leaning towards a minimum of a 30 gallon gray water tank in a size that will fit up in between two beams. I will have to construct a steel cage for it to hang down below the main beams. I may go with the macerator idea to empty it. At the rear of my trailer (since I have the rear bumper dismantled) is easy access to the gray water lines leading in to the 3" bw exit pipe. An inline valve will divert the water to the gray tank. This way I can turn the valve for full hookups and not fill the gray tank. Best of both worlds. The vent for my bw tank is easily accessible, and needs only a T put in it above the tank. I may use flexible abs for much of the piping to wind it around my bw tank.
Still in the design stages, but not that complicated.
