I don't use a monitor, per se.
We've had this rig long enough that I've gotten pretty good at estimating the amp/hrs we've used and I keep track of the total amps charged through the solar charger and the cheap monitor I've wired between the stand alone charger and the battery. Kind of a cheesy set-up but it works for me.
If there's any doubt I just charge the batt to full in the evening and know I've got enough juice to last for 24 hrs. It's a 125 a/hr batt and the most I've ever used is about 75 a/hrs a day when camping in below freezing weather. I may get ambitious and wire up a Victron but it's not a high priority.
With a capable charger and a small battery bank it's feasible to get to full every day and then not have to worry about exactly what your
state of charge is.
