I keep a 12v digital display from Camping World plugged into a 12v power outlet. I watch the voltage. I can see when the charger kicks in because the voltage rises. Then I can use this gizmo when dry camping to monitor the batteries. Cost about $13.
I tried to add a link, but the CW product search is often quite useless. This time too. Shoot, when I got ready to buy a pair of Coleman generators from CW, I couldn't find it via search and I knew it was there.
But I digress....