Normally the factory wiring is either #12 or #14 wire from the fuse panel to the trailer connection. This is good for only a few amps. . It might carry 10 amp at best. Still a 10 hour drive should have recharged the battery from 20% to about 80% full.
You can install a larger wire to the trailer connection, or even change from the standard connection to a 50 amp connector for just the battery charging.
#8 wire will recharge the battery much faster.
An alternative is to install a pair of solar panels on the roof. It will keep the battery full all the time.
SunElec.com has some 12 volt solar panels - look for something in the 100 - 150 watt range, that are small enough to fit in several locations, and still have space to walk around them to the forward sections of the RV for maintenance.
Have fun camping!
Fred.