Truck to 5er DC

[friz]

Plug connecting the truck to the 5th wheel doesn't have 12 vdc (truck side) I think it was pin 7 but I would have to check the diagram to be sure. All other pins have the needed voltage and the brakes, tail lights and running lights are all working when connected. I made the assumption that the 12 vdc pin was supplying voltage to the 5th wheel battery to charge it.
So, what would a 12vdc hot pin actually be used for in the 5th wheel? Is it actually needed?
If I need to put power to it what would be a good source on the truck? There is plenty of 12 vdc around the tail gate/bumper area. It would be a long run to the cab fuses or engine batteries. A little work but buy all means "doable"
For the record we took a 2 month trip last year with this condition. We never "boondocked" long enough to drain the battery. If we had, I assumed the 12 vdc from the truck would have recharged the house battery as we drove if it was hot?
Thanks, Friz