To be honest, I've never heard of air inside a HW Tank being a good thing. My reasons are as follows.
For those of you who own a "Stix-n-Brix" house and NOT on city water, go down to your basement and look at your water supply system. You'll see the pipe from your water pump going into a large blue tank. That tank is an accumulator tank and it's divided internally. The upper chamber is filled with air and set to a pressure specified by your pump manufacturer. The pump fills up the lower part of the tank, then shuts off at a pre-set pressure. When you open a faucet, water is pushed out by the AIR pressure, not the water pump. Once the pressure drops, the pump will kick in again. The accumulator has a "check valve" that prevents pressurized water from being forced out and back into the pump in the well. No where in the water system does the water com in contact with air.
In an RV, there is no accumulator UNLESS you buy a system with one built in like this...
The normal pump builds up to a specified pressure and then shuts off. When the pressure drops a few pounds, it kicks in again. How often this "cycling" occurs depends on the settings of the pump's pressure switch and the type of pipe in the rig. Some pipes expand more than others and act as a "pseudo" accumulator.
Using the HW heater as an accumulator would result in excess pressure in the entire system as the water heats. The heater usually doesn't have a "check valve" to prevent warm water from being pushed back out into the rest of the system, so higher pressures are put on the plumbing connections (especially that gray polybutylene pipe which was banned for home use) and the water pump itself, possibly causing leaks in older systems. Heated water might also damage the pump itself as it's components are designed for cold water use.
Air and water together in a HW heater can be extremely dangerous and might be deadly. How so? The heated water and air could allow bacterium and other nasties to multiply. You could wind up with many water borne ailments, some debilitating, some deadly, especially the longer they breed in the warm water. This is why utility companies pass out "water boil advisories" every time a line is opened for repair or service.
Next, air, water and heat accelerate corrosion. Modern cars have a closed cooling system with a overflow/recovery tank and radiator cap. As the water heats up, it's pushed out into the tank while still maintaining a closed system, as the engine and water cool, it's siphoned back into the radiator keeping the system air free. Unlike a WH, the water in your cars stays there and heat will cause the water to release minute particles of air into the system that are forced into the tank. This explains why you add coolant to the tank as fresh coolant is drawn back into the engine. Since there is always a trace of air, anti-freeze contains "anti-rust" chemicals.
Okies, that's my view on the subject, feel free to kick it around.
Have a good one,