That old of a tank, I would replace it! Or you can have it hydro tested. They basically fill it up with pressure and measure it for leaks. Or they can do a sort of sona gram on the tank, to see how thick the sidewalls are now. Rust on the inside can be much worse than rust on the outside, and both can weaken the tanks.
My work used to have to do sona grams on the air compressor tanks. Those see a LOT more water in them, and rust inside. It is some exploding pressure vessels that caused the State of California to instigate testing tanks back in the 40's!
A state inspector would come in (for a huge license fee for each tank) and test each tank. Back in the 70's and 80's, they would have to shut off the compressed air system, drain the pressure, and open a large fitting on the end of the tank, using special micrometers measure the tank wall thickness in several locations. The new equipment does away with all that - and provides a much faster result, that is fairly accurate.
I would rather see you get a replacement tank from a used RV that was made in the 90's. It would be less than 25 years old. Not over 40 years old. Yes you tank was probably made in 1976 or 77, not 78, same model year as the RV.
Fred.
