I have a feeling it's an attempted patch with fiberglass.
Here are two pics that hopefully help. They're both from '65 trailers and I'm not sure they are the same.
The closet pic was taken by the previous owner of our Caravel
before he removed the interior. The bath pic taken by CJohnson might be the best one for you. Note the tank on the right side with the closet walls removed.
I wish I caught the suggestion to enlarge the tank right into the closet -- if you replace your black tank.