i could be wrong here, but any leakage from the black tank cant be good. if the top isnt sealed you will have sewer gas leaking there instead of venting through the vent.
i just got started this year with our first camper and discovered a similar problem when i tested out tanks. our grey water tank had a couple holes in the side of it.
( note to self...dont buy a camper in the dead of winter when you cant thoroughly inspect it for yourself....)
anyway, i found a simple to use product that allowed me to patch the holes. way easier than replacing the tank and much cheaper. the product is called Plasti-mend, worked great! if you have even basic home repair skills this should be easy for you.