"an air gap in the water heater tank"? I've never done it and never heard of it before, I would think that when pressure builds up and is not relieved from using the hot water then the pressure relief valve would bleed off the excess. It's not done in residential heaters why risk overheating your RV heater because of not enough water?

 

Actually the air void is more or less the top 1.5 inches of the tank. Unless you live in your trailer full time the air pocket is automatically maintained every time you drain your system for storage and then refill the tank. There is no required special method to fill the tank.

As for home systems many have thermal expansion tanks and vertical dead legs to take up pressure fluctuations and minimize hammering. Many of the tanks themselves also have a built in void in the top of the tank that again requires no special procedure to maintain the air pocket.

 

I think Andy's post answers your question of how big an air pocket is in the tank. It is not like 1/2 the tank is empty. As for the relief valve - - it and when the pressure reaches a high enough point it will blow. Trouble is many times the plastic pipe blows first or leaking will occur such as in mine right now. I have to maintain that air pocket. Mine needs replenishing every couple weeks when I am on long trips otherwise I heat only with electric. Besides, I don't want water running down the side of my water heater from a blowing off valve. I have replaced too many electric switches from that on the water heater.

Bob