A vehicle's 7-way trailer socket typically includes a 12V power feed. This is carried on the pin at the 1-o-clock position within the socket, as shown in the linked photo.
This circuit will deliver enough current to the trailer-mounted battery to maintain it while you travel, but not enough to re-charge a depleted one. To actually charge up a battery that is depleted you will need the higher output of a shore power charger such as one of the CTEK units shown on the linked page.
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