It is likely possible, but will require some planning. I added a gray tank to a 1970 RV. As Old Biscuit said, available space will determine what you can do.
Once you figure that out, look for a tank. I found a roto-molding place not too far away that built a tank for me. They do custom tanks if needed, but one of their existing sizes worked (in my available space!) - I just had to tell them where to put the inputs (drain & vent) & output.
You'll need to figure out how to attach/support the tank too, and if any plumbing changes will be needed.
Was definitely some work, but worth it in my case!
CC