Yes charging from your tow vehicle works great!
In fact Lance slide on truck campers use a special connector with larger charge wires for auxiliary (camper) battery charging.
Mine has worked swell for the last 10 years.
A simple continuous duty relay that is on when your truck is running then shuts off when the engine stops does the job. They are called battery isolators and make the system automatic.
Battery IsolatorWire it in like this:
If the main battery is charged and the auxiliary battery is discharged then the main battery along with the trucks alternator will charge your auxiliary battery when you run the truck engine.
Charging is based on voltage. Dead battery's draw down the system voltage increasing current flow into the discharged battery.
Note: A regular 7 pin trailer connector has all the same sized wires limiting charging current. Using larger gauge wires in the charging circuit will improve results. Notice the red lines in the diagram.
You can leave out the surge protector and the rocker switch.
