i assume you have at least one vent through the roof, for your sink, right?
I suggest tying the at least one of the tanks into that, with perhaps the other on an AAV, to give you added air relief for smooth draining. You simply won't have enough relief with just a single 1-1/4" vent for a big ole 3" drain line. AAV's come in various sizes, so just size it right for the job, whether it is for the black or grey.
The reason you want the AAV to be ventilated and accessible, is that the seals can go bad after a few years. Just to ease your conscience, you'll know an AAV is going bad when smells come into the interior. If that happens, simply cut off the old AAV and replace it.
Another thought for running the tank vents, is with flexible PVC pipe. It makes it easier to route along the curves of the airstream... just make sure you still have positive drainage for the vent pipe. Flexible PVC can also be glued, cut, etc. as normal Schedule 40 PVC pipe, and it is fine for DWV systems. Here's a link:
http://www.plumbingworld.com/spaflex.html
christopher