Could be a bad seal or more likely some trash, perhaps toilet paper in the seal. Clean the tank well then Empty the tank, Then run a hose with a jet nozzle up the drain to flush the valve area. Do it just before or during a hard rain so any minor issues flush away or take it to a car wash and do it there. While flushing, cycle the valve several times. Then adjust the valve throw so it fully closes and fill the tank with fresh water and see if it leaks. If it does try again. No luck? the seal is damaged and may need to be replaced.
A good way to prevent TP from causing an issue is to dump the tank first thing in the morning before any use and then leave any additional morning contents plus a couple gallons of extra water for the next stop. The sloshing will break up the TP in just a few minutes.