You guys totally rock!!
Just emptied all water from tank, pumped air pressure to 26PSI (2
below the cut-in pressure of the pump) and turned it all back on.
Now I get 8+ gallons before the pump comes on instead of only 2!!
Even the missus says we've always had the pump kick on after each
flush, so maybe it's been 12psi since we bought the house 4+years
ago. So, I'll keep an eye on the air pressure from time-to-time to
see if/how it lowers. If it doesn't go down any time quick, I'll
conclude that the tank is fine.
Again, thanks to all of you!!