It was in the early 70s that gray tanks became mandatory, so yours should have been equipped with one. That's assuming that the rig was originally designed and sold as a travel trailer and not for some other purpose.
Our 1967 Airstream does not have a gray tank, so it's necessary to bring the blue tank (a portable gray tank in our case) or to have full hookups or to use a plastic tub in the sink and dump that into the toilet. (The rig is designed so that we can divert gray into the black with valves, but it's kind of messy, so we don't use that method.)
:)
Lynn