We did and are happy we did.
We use the black tank as an additional grey tank.
We use one of these
which adds an additional valve before dumping. So if we are somewhere for a long time and one of the grey's fills up we leave that closed (so nothing comes out the hose connection) and open the grey that is full and open the black and some of the water that is in the grey flows over to the black, equalizing the tanks. We then close their respective valves and voila we can stay longer.
Now you should note that we did this on a brand new trailer that we NEVER used the black tank on. If you have used your RV's black then you are contaminating the grey tanks with waste. Probably not a big deal but you can never just dump your grey water on the ground (where it is legal...which is only a few places anyways) as it is now black water.
The main parts of the install are:
1.) Finding a 12v line to tap into to power the fan
2.) Running the vent hose somewhere to vent it outside...we ran ours to the black tanks vent pipe and tapped into that.
3.) Capping the black tank.
Feel free to hit me up with any questions...we bought our trailer in Feb. and it was put in on day 1. We have full timed in our trailer since May 1, so it has been our main toilet for 4 months now.