Did 1984 13' Scamps have grey water tank?
I'm guessing Scamp never even thought about putting a grey tank on a 13'er until they added the shower option (early 2000's, maybe??). Then, of course, they had to.
Now, some order it as an option even without the shower. No harm, but an external tank is acceptable in all but a few places. The choice comes down to whether you'd rather use a dump station or haul an external container to an acceptable disposal location. It's more common to require an external tank be a sealed unit. There's some sense in that, as an open bucket can attract unwanted critters.
My family traveled all over in a tent trailer in the 60's and 70's. No grey tank, and we did the garden hose and bucket thing. When it was full, we found a tree and gave it a drink. Times have changed.