Agree. The trailer simply cannot draw enough amps through that long run of small wire to drain a vehicle battery that is inches from a 140 amp alternator.
A camera or some phones will not draw down a running vehicle battery either.
In the meantime, you might can just disconnect the charging wire that feeds the trailer battery, or pull the fuse if the trailer has one between the battery and the 7-pin plug.
My trailer has solar, so I rarely ever have the vehicle charge the trailer.
Whatever the guy in the above post was talking about with the alternator and the C pin might be the best place to start.
To run a battery down takes some serious work, so the alternator must be at fault (not charging). When alternators die the ECU (trucks computer) and the blower vent fan will run the battery down in 30-45 mins, so your alternator must be putting out some charge that you can go 3-5 hours without the battery being dead.
The camper's battery could possibly contribute voltage back to a dying vehicle battery, but not significant through that long run of small gauge wire.
None of this helps any except to help you eliminate some possibilities and push you towards looking at the alternator first.
