I was considering a nice flush mounted voltmeter to monitor the house battery. I even bought one which didn't work (got my money back). I eventually bought a portable cigarette lighter voltage monitor to plug into a cigarette lighter connected to the house battery.
The advantage of this unit is it's portable, and can be removed if the plug needs to be used to power something else. It doesn't require modifying the Systems Monitor, or cutting a hole to mount a more permanent meter. It can also be plugged into the dash cigarette lighter to check the chassis battery charge/status.