Gregg, something I do not understand. You said the sub-panel is fed from MP via 3-wire, and the SP has isolated neutral bus. I assume by 3-wire you mean L1, L2, & N with no EGC. If that assumption is true, then your meter is correct, you do not have a ground.
If you run 3-wire, meet the conditions of 250-32 ( I think that is the code reference)of no metal paths, then you need a N-G bond in the SP. Pesrsonally I would always require 4-wire to be on the safe side.
