If you have black water backing up into the shower I have a question. Do you have the black tank valve open but the sewer cap on? If so the reason that you are getting black water in the shower is because the black tank is full to a level as high or higher than the floor of the shower. The filling of the black tank with grey water this way is a reccomended way to use the kitchen or shower, but capicity is limited.
On a 73 I do not think you have a gray tank.
If you are getting black water in the shower with the dump valve closed then I would suspect the valve is a leaker.