The refrigerator on gas draws +12VDC from the battery for its control circuit. Don't believe me? Disconnect the battery and try to run the fridge. There are other +12VDC parasitic loads that are there whenever the battery is connected to the trailer - the propane detector for one. The truck and trailer are wired with a +12VDC feed for the trailer battery(ies) for a reason. Why disconnect it? If you're so worried about drawing the truck batteries down, install a battery isolation relay to break the feed whenever the truck isn't running.
Rusty
