Here are some shots of my Battery Meter Mod for my 189 FBS. I had some of the parts already, but the rest I picked up on eBay. All-in-all I have about $35-$40 into it.
Here is the meter. The DPDT Center Off switch toggles between volts and amps:
Here I am pulling 4.5 amps. I have the furnace running, the TV on the inverter, the power for the Jack antenna, the radio, as well as every single light fixture (all bulbs are LED):
Here is my current battery level in volts:
I wired the 100A/75mV shunt at the battery and ran 5 conductor furnace wire from the battery to the meter (this was the most time consuming part). I cut the holes for the switch, fuse, and two panel mount meters into a standard blank switch plate. I then mounted that to a Panduit/surface mount box.
I figure I will use it as is for a season and see what happens. If I'm not getting what I want out of it, I can then drop the $160 into the TriMetric system. But for the time and money I have into it, if seemed to be a logical first step. Heck - the hard part with pulling the wire is done - I can reuse that if I swap it out.