Smart Charger Voltage 15.9 volts ? (VEC1093DBD)

The 1093DBD is temperature compensated on nominal 14.8v at 77F. My chart only goes down to 32F, but that would be 15.3v equivalent, so 17F would be higher than that. So your 15.9 seems reasonable.

However, the RV fridge control high limit is 15.4v and that is not temp comp, so you raise a good point to not use the charger when it is below freezing unless you disconnect the batteries from the Rv first, just as you would if equalizing in mild temperatures.