My trailer frame and under-floor plumbing were done at Colin Hyde's shop. He used the grey tank from VTS
. You can see pictures of the installation starting at post 185 of this thread
My trailer had a heater at the forward end of the galley with a vent above. I repurposed the vent to be the grey tank vent. As you can see in the pictures, the tanks are close to the axles for good weight distribution and are separated by a full bay. There is a PVC pipe coming out of each one, joined by a flexible Fernco
There is one drawback with this method -- it drains very slowly. If you're in a rush, you can't empty the tanks in less than 15 minutes.