IMHO, use the genset with a portable battery charger, and leave the disconect switch in the off position. No sense to plugging the trailer into genset to operate everything else when all you want to do is charge batteries. Built in chargers/invertors would take a long time to charge battery, designed to keep batteries up when using equipment, not to charge a low battery. Make sure your portable battery charger matches your battery type, and once per month should be sufficiant to keep batteries up, charger will tell you when it is done, no guess work!