Hi ozark.flyfinger,
There are a number of variables including ambient temperature and humidity, what's in the refrigerator, how you have it set, all of which add up to more or less run time for the compressor. The more the compressor runs, the shorter the battery life. Other 12V systems can also put a load on the batteries. For example, running the furnace.
Most people find that if they're towing for 6 - 8 hours, the refrigerator will run for an overnight stop without shore power.
It's a good idea to disconnect the cord to the truck. If you run the trailer batteries down, you don't want to also have dead truck batteries in the morning.
If you're thinking about boon docking, you'll need a way to recharge the batteries; probably a generator or solar installation.