If your tow vehicle has the Pollak RV-style 7-blade connector, the pin outs are as shown on the attached photo.
With your tow vehicle's engine running, you should have +12VDC from pin #4 (+12VDC) to pin #1 (ground). Some vehicles keep pin #4 hot at all times, while others utilize a battery isolation relay to disable pin #4 when the engine isn't running - this keeps any load on pin #4 from discharging the tow vehicle battery if the trailer is left hooked up to the tow vehicle for long periods of time when the tow vehicle isn't running. Therefore, it's best to check for the availability of +12VDC with the engine running just to be sure.
If you have +12VDC on pin #4, your trailer battery should charge from the tow vehicle. Be advised, however, that due to voltage drop and small wire size in most tow vehicles, the trailer battery probably won't charge rapidly from a deep discharge condition - it's better to think of this as a maintenance charge capability.
2016 Ram Longhorn 3500 Dually 4x4 CCLB, 385/900 Cummins, Aisin AS69RC, 4.10
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