Yes, the 12VDC batteries are recharged by the truck alternator via the +12VDC feed in the trailer umbilical. This is important to us as our RV has a residential fridge that runs off a 1500 watt pure sine wave inverter when we're towing, so the truck's charging system keeps the RV's batteries topped up.
Depending on the size of the 5th wheel, the cost and the size of the inverter supplied (if applicable - most 5th wheels won't have inverters unless they have residential fridges), you may have 1 each 12 volt battery, 2 each 6 volt batteries wired in series, or 4 each 6 volt batteries wired in series/parallel.
Rusty