Many times as Pulsar suggested this will stop the leak:
1. Turn off the water heater.
2. Turn off the cold water supply line.
3. Open a faucet in the RV.
4. Pull out the handle of the pressure relief (P&T) valve and allow water to flow from the valve until it stops.
5. Release the handle on the P&T valve - it should snap closed.
6. Close the faucet and turn on the cold water supply. As the tank fills, the air pocket will develop. Repeat this procedure as often as needed to reduce the frequency of the weeping P&T valve.
