ideally you needa run the generator til its back to 100% (until you have solar to finish the job), for a set of golf cart batteries thats much longer than 3h.. IIRC it takes 8-12h to recharge a pair of em from half depleted, and unless you plan on driving that much every time they need recharged this is why a generator will save your batteries from an early death.
however.. driving to recharge is largely applicable to RV's only, since your trailer battery is so far away from your alternator and connected through a trailer plug.. its actually going to take longer to recharge it driving because I really doubt your trailer battery bank seeing more than a few amps of power from the tow vehicle.. you can drive for >12h and still not have a fully charged battery bank if all your getting is 4-5A for the bulk charge.