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The grey tank needs a vent, just like the black tank.
The grey tank has gases build-up, similar to that of a black tank, and therefore needs to vent. Also, when you drain the grey, the backpressure will siphon out the the p-traps, allowing smells to escape. The tank will drain faster with a vent, because it doesn't have to gurgle or siphon.
According to the 1972 AIRSTREAM service manual, grey tanks have a separate 1-1/4" vent from the black tank. That being said, most people dump one at a time, so it may be possible for you to combine the grey and black vents into a "vent stack", so as not to add another hole through your roof.
I suggest you go to your local hardware store and buy some "spa-flex schedule 40 flexible pipe. It bends really easily, conforms to the curves, and uses standard ABS fittings. Use a standard 1-1/2" slip-fitting, with "eternabond" to seal it against the tank, to connect the new vent to the existing grey tank. Then attach a reducer fitting to go from the 1-1/2" slip to 1-1/4" pipe. Snake the spa-flex over to the black-tank's vent pipe, and cut it into that pipe by adding a wye fitting.
Be sure not to allow the vent to run perfectly horizontal... it needs to allow positive drainage back to the tank, this is for both sewer gases and moisture.
In case someone suggests it, do not just use an Air Admittance Valve to vent the grey tank. Though they are easy to put in and will help the tank drain properly, it won't allow gases to escape... they will just build-up.
christopher
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