You probably shouldn't connect the two tanks with a single vent because the drain lines are likely 3", but the vent is probably only 1-1/4".
Depending on how you built the drain lines, there may not be enough relief air through the single roof vent, and the draining will not "swirl" in the tank as it drains. This will eventually cause grime to build up.
You may want to think about venting the gray tank separately with an air admittance valve (AAV). Install the AAV at a height 4" higher than the highest drain, your sink, so if you fill up the 12 gallons in the gray tank, and your drain lines start filling, it will flow up into your sink before the AAV. You have to install the AAV in an area that can get air, and be accessible for maintenance if necessary, but it is a lot better than cutting a hole in the roof when you are trying to find ways to vent the drains.
Plumbing vent codes are available online, look some up if you aren't familiar with their specific requirements such as elbow types, horizontal venting, riser types.
Here is info on AAV's:
www.rectorseal.com or
www.studor.net
enjoy,
christopher