[Motor31]

Actually, given the design of the RV water heaters, air in the tank is a good thing. There is a check or anti back flow valve on the inlet line to the water heater that prevents pressure from backing back into the water line, pressurizing it to beyond it's capacity and creating a leak. It also prevents the water heater from trying to heat the water in the line outside the heater so that it is not constantly running. That same check valve is the one that create a humming sound when it starts to go bad as you use the hot water. There is a similar valve on the shore water inlet connection point to prevent the water pump from dumping water out of the rig when you turn it on while dry camping. The RV system is quite different from the boiler in a naval ship.

The suburban manual I have for my rig on page 6 specifically states to prevent the dripping or weeping from the pressure relief valve you should allow an air pocket to be formed inside the tank. The instructions state; "This air pocket will form in the tank by design. However it may be diminished by use". They follow that statement with instructions for reforming the air pocket. by turning off the inlet water, opening a faucet inside then opening the pressure relief valve until water stops flowing out of it. At that point close the valve, faucet and open the water line in that order. Doing so will reduce or eliminate the weeping from the valve as the water expands due to heating. An additional option is to install an accumulator tank.