We don't boon dock & I use the generator when not plugged in.
Yup just get two 12 volt battery's if thats whats in there now.
Get agm this way you do not need to check water or anything like that.
If there are 2 six volts there just put 2 of them back in there as the wiring would be the same.
if you dont go agm then you will need to check the water often.
we dont know what you can fit so its hard to give you links.or whats in there now...
advanced auto here. lead acid
https://shop.advanceautoparts.com/p/autocraft-marine-battery-group-size-24m-550-cca-24dc-1/2080060-P?navigationPath=L1*14920%7CL2*15000%7CL3*15601sams club has them for 59 bucks
https://www.samsclub.com/p/duracell-marine-battery-group-size-24dcc/prod3590221?xid=plp_product_1_8here is a grp 27 agm.
https://www.samsclub.com/p/marine-27magm-18-mo-free/prod21234614?xid=plp_product_1_3