I don't know how to break it to you, so I'll just come right out and say it. Your 66's gray water tank is so small, you could take the whole trailer apart and not find it.
In other words you do not have a gray water tank.
the issue of draining gray water on the ground was not considered taboo as it is today. the grey water plumbing joins the outbound pipe between the dump valve and the bayonet fitting for a hose or cap. You are fine as long as you have a blue buddy or a sewer connection.
Adding one can be done, but frame integrity and the engineering of the added tank must be taken into acount. It is usally simpler to just get a blue buddy and tuck it under the trailer.