House meter says 9v DC but individual breakers all read 12.5

[Goodwake]

I have a 2006 3600 Express
Whenever the 70amp cabin breaker is ON the voltage indicated on the "DC system monitor panel" drops to 9v on both the house bank and the engine bank.  Engines spin over easy, battery charger works its way down like it should.  All the DC breakers have 12.5 v measured individually with VOM.  Can someone tell me what to look at next?

[Craig352]

Just bought a 2016 3600 Open and it's reading exactly the same even though I know I'm getting full voltage. Love to hear more from someone who knows.