I am not Moomba but outback extreme does sell tanks. I bought mine local many years ago. As far as a blow down valve. They also sell those but they are not automatic. It is spring loaded, has a ring on the end of it that you can just pull on and it empties the air. I placed my valve on the bottom of the tank and while I don't really do it that often I can just reach up under the truck and pull it. What I do on a more regular basis is when I disconnect my air chuck from the quick connect is as I start to pull it apart I let a pretty fair amount of air out which also expels some moisture certainly not all of it but probably the majority. My tank is at least 15 years old and still hasn't rusted through yet from the inside and the outside still has plenty of original paint on it after living its life under the truck (though I do not live in a salt state)
