You would need a huge charger in amps size to do those 8- 6v batts at 20% rate if you got down to where you needed a Bulk stage in your recharge.
With AGMs it is vital that you have a charger that can hold 14.x volts steady in the Absorption stage while amps taper to 0.5a /100AH of bank. And you need an ammeter or monitor that can show when you get to those amps and know your batts are truly full.
You will need to be on shore power to do that. If you have shore power, you don't need all that. If off-grid , you can't ever get them properly full, so they will die an early death.
Something is not logical here. What is the actual scenario?
Once we know that, then we can give you some useful advice. (For free! :) )