If you need to charge your TWO 6VDC batterioes in SERIES to a 90% charge state in a quick three hour time span the 15AMP charger may be a bit short...
Battery science says you need 14.4VDC BOOST MODE and between 18-20AMPS DC current available to re-charge a deep cycle battery to its 90% charge state in a three hour period.
You will be able to re-charge your two 6VDC batteries ok using a smart mode battery charger but may take a bit longer.
I'm always at a spot where generator run times are limited by rules at the camp ground so this is very important for me to get my batteries re-charged as quick as I can if I am using the generator for 120VAC to run my on-board converter/charger unit or a portable smart mode charger.
Roy Ken