upgrade converter/charger on 2015 Keystone Passport 2920BH?

[FreeFromCube]

First time poster! :-)

I bought a 2015 used 2920BH in the fall. It's in storage now, but I'm planning out a battery upgrade, and potentially solar. It currently has a single 12V battery, and I plan to upgrade to 2 x 6V 200+ Ah deep cycles connected in series. I don't plan to do long term boondocking, just occasional overnight unserviced stop while on the road and my plan is to have fresh water and full batteries to make the stop more convenient.

Does anyone happen to know what the converter/charger is in the trailer? If single stage, what are good options to upgrade to multstage? I'm guessing what is there isn't great for larger 6V golf cart batteries.

I'm also looking at some solar and charge controller, but that's for another day :-)

Thanks!
Mark