[JRTJH]

Sometimes the factory fails to properly glue the vent stack pipe into the top of the tanks. That "might" be a source of a leak if the black tank was full. Another possibility is a "split tank" which doesn't happen very often since manufacturers started using "rotocast" tanks.

You didn't say whether the leak you saw was "smelly, black tank effluent" or not, but I'm guessing you've determined that the leak was from your black tank? Usually, if you notice the leak, the underside of your trailer is contaminated since the coroplast seals that area and any leak would be above the coroplast. You may want to do some "snooping" to see whether you've got a "big mess to clean up"....