My brother in law just bought a 39ft 5th wheel. After bringing it home without any problems, found that the 5th wheel batter was dead. Before long we discovered that the trailer (7 pin) was not giving any power when plugged in.
I looked in the truck manual and it has a couple of fuses relays that sound like they should be checked.
-Trailer tow brake controller, Trailer tow battery charge relay coil (10A fuse)
-Trailer battery charge relay (30A fuse)
-Trailer tow battery charge (relay)
Could it be one of these or all. The brake, parking and turn signal lights all work fine. Just the charging of the 5th wheel battery.
His truck is a 2009 F350 PSD with the tow command package.
Are you referring to the battery that runs the camper when not connected to shore power? On my camper, that battery is not charged by the truck. But I do have an older unit. Maybe the newer ones are wired differently.