We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "rburr49"
saying something like:

Is it possible that the tank itself (copper with that dry foam
insulation stuck all over it and about 20 years old I imagine) has some
kind split/hole in it?

Old tanks sometimes develop leaks from the bottom where the metal is
stretched thin during the stamping process.

Is there anything we could use like radweld (?) to seal it up? The
thought of having to replace the tank (meaning getting a plumber in to
do it) is a bit of a nightmare cost-wise!

Not much. I've tried this before (solder or fibreglass) and quite
frankly it's not worth the hassle - it always leaks again.
Replace with new.
--

Dave