The Link 20 (and other battery monitors that I know of) use a shunt in the ground leg of the battery bank to monitor current into and out of the bank. Any ground lead connected to the battery side of the shunt can steal current from the battery (or add charge to it) without the monitor being aware. The only thing that should be connected to the battery side of the shunt is the battery.
No twisted pair from the house shunt to the monitor? Where is the L20 getting it's current info for the house bank?