Your battery/s in the rig will go dead in short order if not charged continually or disconnected from the inside 12V system. There are a few parasitic draws like alarms, circuit boards, radios, etc. that will drain a battery. If your towing vehicle didn't keep the battery charged, it is like others posted, a relay or fuse needs to be added to the vehicle's distribution box. Check your owner's manual.
BTW, a hand held electrical meter will serve you well for diagnosing electrical issues in the future. Every RV'er should have one.