Things that your tow vehicle wiring must power while towing (in no particular order) to prevent draw down of trailer battery:
- clearance, tail, and brake lights: typically 1 amp/bulb, 8 amps total on my little A-frame. Brake lights add more, but are not on continuously. Running at night with lights on makes a big difference.
- parasitic loads of camper (alarms, stereo when off): 0.75 amps on my little A-frame.
- inverter: 13 amps with inverter loss and fridge is only AC load
- recharge camper battery: 3 amps if batteries were fully charged at start, and not much voltage drop
As you can see, you would have 24 amps through the 7 pin at night, which is going to cause a voltage drop. The whole 24 amps has to go through the ground (common) wire even though the lights, brakes, and other loads are supplied through separate circuits and wires. Your tow vehicle wiring harness (and the harness on the camper from the connector back) would have to be 10 gauge wire, at least on the ground wire. The voltage drop in the harnesses will cause camper battery charging not to happen, and the needed current to be drawn from the camper battery because it is at a higher voltage than the harness.
A way to check: hook up your tow vehicle electrically, turn on all your running lights, and your fridge/inverter on. Start the motor in the tow vehicle. Measure your camper battery voltage at the battery before you hook up, and then with everything running. If the "everything running" voltage is higher, then you are good to go, and the tow vehicle is carrying the load. If the "everything running" voltage is lower, the camper battery is supplying power.
You can run the same test at the inverter input, and use the disconnect switch to switch the camper battery in and out. If the voltage with the camper battery connected is higher at the inverter, the camper battery is supplying some of the power.
Hope this helps and makes sense
2014 Rockwood A122 A-frame
2008 Hyundai Entourage minivan