Ready to throw a mattress in and go camping??? That owner is obviously stretching it way beyond the truth. The plywood floor is visibly rotted with large missing areas of it. You should not walk on the fiberglass shell without new plywood installed.
But someone with high energy and a strong back can sort that out in a couple of weekends with a hundred bucks or so.
As to sorting out the cause of the leaks
...anyone's guess as to how much time that will require or how much money. I had to redo my windows
with new seals throughout them. I had to replace the gasket on the door, put a cap over the door to divert water, replace the wood frame and canvas and the bulb seal at the bottom of the popup top. I also had to patch in some fiberglass where there were numerous holes such as where the ledge for the seats was attached to the shell. Also I had to remove some rotted floor and patch in a new piece. I have put in lots of labor time and also money for materials just to fix the basic issues with the structure so I could begin to get ready to put in a new interior.