Hi Paul
I looked at the SBIG field power supply and for peace of mind you should rather use this.
12VDC Power SupplyThis optional power supply converts 12VDC from a battery to the 5VDC and 12VDC required by the camera. It is for operating your ST-7/8/9/10/2000 camera from a battery in the field, or anywhere there is no convenient AC power. Part # 50388
I dont know what the cost is but this power supply does not use an inverter.
It will use voltage dividers and regulators as it only needs to convert DC to DC.
Regards
Steve
Another reason for suggesting an inverter was because you could use it to power other accesories (such as laptop etc) as well as the camera. Inerters are not as risky as some would suggest unless it is heath robinson.