I carry a back-up battery charger with me in case the built-in converter/charger goes out when camping...
I use the the B&D VEC1093DBD model which is a 40AMP Smart Mode portable charger setup. I run it off my generator when needed to be used. Each battery in battery bank will need to see around 20AMPS DC current when hit with 14.4VDC Boost voltage if you want them to get re-charged to their 90% charge state in a three hour generator run time. To get to the full charge state this will take over 12 hours to finish which is too long a period for me to run my 2KW Generator setup out the woods...
I think you can still purchase the VEC-1093DBD... This is a real popular battery charger amoung the RV guys... Got mine from AMAZON a few years ago...
My on-board converter/charger to keep my three batteries working in my POPUP trailer battery bank is a PD-9260C 60A Smart Mode charger setup. I run this off my 2KW Generator when allowed to run my generator in the woods each morning for three hours.
Roy Ken