Having searched high and low for a mobile charging solution for my MBP, I finally accepted that there is no 12 volt adapter for the MagSafe 2, so I purchased an iTec power inverter from WestWorld Computers in Edmonton.
Price was $44 + GST.
The inverter is supplied with a 12 volt plug, rather than the bare wires that some inverters are equipped with, and it is designed to drop into a cup holder. The outlets are on top and number two 110 volt, one 12 volt and one USB. The USB outlet does not give sufficient power to charge an iPad. It handles the iPhone with no problem, however.
Total supplied power is 200 watts and the instruction come complete with a chart so it's easily possible to check that it's not being overloaded. The unit is fan cooled, and while the fan might be considered noisy in a car, it is barely noticeable in a truck. There is a green light to let you know that all is functioning correctly, and a red one to indicate faults. The unit can be turned off when 110 volt is not required, and will still allow 12 volt and USB to be used.
I presently have my MBP plugged in to one of the 110 volt supplies, my Garmin GPS plugged into the 12 volt and my iPhone 4 plugged into the USB. The MBP is also charging my IPad 2 via USB 3.0. The iPad is running MotionX GPS HD at maximum screen brightness, and the MBP is running iTunes through the truck
So far, so good. The iPhone and iPad have charged to 100% from around 70%, and are holding the full charge. The MBP was at 100% when I plugged it in, and it is holding that charge.
Note that this unit can be used only on 12-volt negative earth systems. Unlike European trucks which have 24-volt systems, North American trucks all have 12-volt systems. Positive earth is about as common as hen's teeth so it should operate in most cars and RVs.
Hope this mini review is useful.