You just filled your RV with SEWAGE. Once the tank is full it fills the vent pipe and once that is full it boils over the top of the pipe to run back down the outside of the pipe and out the underbelly.
Look on the roof your vent pipe is not sealed it just vents under the rooftop vent. Your tank fitting did NOT leak. If it was not tight you would have POO JUICE constantly weeping traveling down the bumpy roads with POO in the tank.
To avoid this EXACT problem as it is VERY easy to get distracted I set the timer on my phone for 10 minutes with a LOUD Dog Bark when I close my tank valves and hav the flush system activated.
Just wait for the SMELL once the heat cooks the POO. If this happens you may want to file an Insurance claim.