If the solar was ever not working as I desired, or just not enough juice, I wanted a way to be able to charge the trailer battery from the vehicle alternator (as the 12v 7way connection wouldn’t really be enough) so I did this:

• I added a female ‘Tow Motor’ connection to the rear of my vehicle.
• Ran 8ga wire all the way to my battery, but put it behind a solenoid, so it was disconnected unless I wanted it connected and the car was running (would be bad for my trailer to drain my vehicle’s battery overnight)
• Connected one of my Aux Switches (I set aux3 for this) to flip the selenoid, and made sure if vehicle was off, it wouldn’t stay on.
• Ran a male “Tow Motor” connector on my trailer directly to the batteries on it.

Now, when I connect my trailer, I just connect the 7way (for all my trailer lights, blinkers brakes, etc), and this “Tow Motor” connection.  If I want my alternator to also charge the trailer, I just hit the aux switch on my dash, and it connects the trailer to the car battery and hence alternator.

THE ONE MISTAKE PEOPLE MAKE IS DIRECTLY CONNECTING TO THEIR CAR BATTERY WITHOUT USING A SOLENOID.  If you do this, when you wake up one morning, you might have a low trailer battery, as well as a low car battery, and be unable to start your tow rig.

You definitely need a way to disconnect your trailer from your car battery, like I explained above using a solenoid.  You have been warned. 🙂