I’m not aware of any plug or plug option from Bennington for charging the batteries. A good ”smart“ charger (I’m partial to CTEK) will do the job. I hooked mine up to the port side battery, as it seemed to give me more room for the charger in that area under the seat. I‘m not expecting the stock batteries to last long, given the fact I haven't heard of the manufacturer on the battery sticker.
I‘m far from an expert on the topic battery charging of dual batteries, however my understanding is that Illinois Ave outlined the best way to charge batteries wired in parallel. Based on my limited understanding, the way I do it with one charger hooked up to only one of the batteries will charge that battery fully first (and the second battery will only be approx. 80% charged). However, I was under the impression the second battery would be brought up to full charge in float mode on the smart charger. One drawback to this method if my understanding is correct is that the battery life may be reduced. (There may be other drawbacks). I’ve used my method before on a previous boat and I never had any problems, including expected battery longevity. You also want to make sure that you replace both batteries at the same time and not to mix different types of batteries.
Two batteries are connected in parallel when the selector is set to 1+2. Both batteries attempt to attain equilibrium when connected as such. Attaching a charger to either would ultimately charge both (assuming both batteries are in relatively equal condition) given the relatively low impedance of the of the battery cables. In the event one battery won't take a charge as well as the other, I'd say it's best to charge them individually to allow max total charge.
The best option is a 2-bank battery maintainer so each top off and float independently, plus both batteries are disconnected from all loads.