The air pocket in the water heater may need replenishing.
The relief valve will tend to leak without it. The following is from the Atwood manual.
"To replace the air pocket start by turning off the water
heater, shut off the incoming water supply
and open the closest hot water faucet. Pull the
handle on the pressure-temperature relief valve
straight out to allow the water to flow from the top
part of the tank. Once the water has stopped
flowing allow the handle on the valve to snap shut,
close the faucet and re-engage the water supply."