Since my Magnum 4448 Inverter does not have an auxillary 12 volt connection, it had to come from my Schneider Connext MPPT 60. Luckily it has an aux port that can supply anywhere from 5V to 13V.
This is menu driven off the MPPT 60, and there is different triggers, low voltage, high voltage,etc. In my case I want to set it up for High voltage as this will be when its charging and batteries will gas. Say...54 volts or more and the fan will run. The battery connections from the MPPT 60 will have to be connected to sense the current voltage of the battery bank.
(51.1 volts to 51.2 is a fully charged 48 volt battery bank)
Power Vent Documentation