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Way late to the party here, and don’t even have a truck camper (have a Bigfoot 21 ft trailer) but newer vehicles use a computer controlled alternator that does not raise the voltage very high but for a short time, then drops it back to well below 14 volts. Most old school auto mechanics are used to an alternator running at a steady 14.2v or so, but to conserve power (and the batteries last longer as a result) the computer of mid 2000’s and up vehicles regulate at a much lower voltage. This coupled with the rather small wire supplying power to the 7 pin results in no measurable charge of a battery connected to it.
I truly think that the sole intention of the charge line is to provide the small amount of power needed to charge a small sealed battery used in emergency brake systems on cargo and utility trailers.
If you want to charge camper batteries from a newer vehicle, you need to have a DC to DC charger installed in the camper. Then you run heavy gauge (4 to 2 gauge depending on the size of the charger) from the truck batteries to the rear of the truck, and connect to an Anderson connector. Then you plug the camper up and the DC to DC charger has more than enough current to work with. DC to DC chargers will have a four stage charge program in them similar to many solar units or good power converters such as Progressive units. In addition, the DC to DC has buck/boost capability to raise or lower the current to the desired program levels of the charger. Basically you give it something in the 13v range and it does the rest. These units are built in 20, 40 and 50 amp and I think there may be a 60 amp DC to DC charger on the market. They will typically draw about ten amps above their output due to losses in the buck/boost of the current, and also have lithium charge profiles built in also. They prevent the lithium battery from sucking the power from the truck rapidly, which lithium would do if not restricted in some manner (which would destroy the trucks charging system).
Also, the DC to DC units will not draw power when the vehicles voltage drops below about 13.2 or so. This way, when you shut off your motor, the truck batteries drop in voltage to a resting state of generally about 12.8 or 12.9 and the DC to DC charger will automatically disconnect to prevent discharge of the truck batteries. You can add a automatic battery separator on the trucks feed to the camper if you wish to safeguard the system but its probably not needed.
Australian’s have been doing the DC to DC charging for some time now, if not having pioneered it. Redarc is one of the major players in this market with some very nice units that are environmentally sealed, and Renogy also makes similar units. I am sure there are others, but am not familiar with them.
If installed, you would want to go into the camper and and find where the seven pin wire enters and disconnect the power input and secure it, so it is not attempting to charge the system in opposition to the DC to DC charging.