the reason you weren't getting a reading at the stat was there is no common if the stat only has 4 wires, you'd have to find common inside the AHU and meter it there.
Well.........in older units there's normally enough load on the switched wires (white, yel, green) that you should be able to read from the red to one or more of those. That will at least tell you if you have transformer power, and that the unit has power. You must of course do this with blower off, heat and AC off
I chased my tail for awhile one time, a guy named "Richard" had added an extension to the stat wire AT THE TIME the house was built, and went from conventional 4 wire to a random chunk of 5 or six wire, wire nutted inside a wall.
I finally got wise, and found the entrance hole under the floor, and you could see the nuts UP in the hole. Would not have been so bad if he'd left 'em in the BASEMENT.
I was lucky..........there was no fire barrier in the wall from the floor to the stat, and I was able to pull a new stat wire.