Airstreams did not have a grey water tank in 1968
. That started about 1973, we didn't have one our 72 either. I'm not sure but I would think that in 68 they at least had a blackwater tank.
I was so Airstream stupid I didn't even know where to look for our blackwater tank. turns out ours was ABOVE the floor directly under the toilet. Look there. Everyone was so patient here with me, bless them.
Freshwater tank is probably up front of the axles under the first 1/3 of the floor.
I don't know how to link it here but if you go to the Tradewind section of this forum and click on "Annettes Folley" you can see how we fitted a grey water tank into the back of our tradewind. Hope I've been some help.