the 7 pin cable connection next to the hitch that connects the truck lights and electric brakes to the trailer- how do you get 12 volt power so my truck will charge the trailer batteries while we drive?

If it's a factory 7-pin via the towing package, you should already have +12V there. Can't remember which pin off the top of my head,, but it should say on the flap you lift to insert the trailer plug.

 

Good way of remembering where the constant +12V line is that it is directly across from the ground pin. The ground pin is closest to the tab (and is not the brake pin). Use a meter, or at the minimum, a continuity tester to verify this.

Open up the trailer connector and see if that line is wired in. If conventional colour code was observed, it will be a black wire. If it isn't, well, add a wire to the loom. If it is, trace it back to wherever the cable goes. You can pull approximately 30 A off this line. Use a battery isolator diode block to connect it to the trailer battery system.

 

In my 2010, I checked the 7-pin connector to be sure it would provide power. It does - but only when the key is turned to "On". Otherwise, no power.

That should charge the batteries in your trailer depending on how it was wired. You can check by disconnecting the battery in the trailer then checking for voltage at the wires that would connect to the battery.

 

Thanks everyone. I checked the cable socket, and there IS power there. (And there is a handy diagram on the socket cover that shows what each terminal is). What threw me was the difference between the truck battery (13.7 or so with the engine running), and the trailer battery (about 12.5), even when all is connected and the truck is running. Seems like they would be equal, but maybe the two large trailer deep cycle batteries will take a while to come up.

Anyway, I'm pretty sure my truck alternator is going to charge my trailer batteries on a longer trip.

Don