Good point to raise, and I am glad people are thinking about this. As for a truck you have to remember something about electric motors, they have LOTS of torque, which is what matters when you tow something. Also unlike a conventional gasoline engine the torque in an electric motor is available instantly as opposed to having to wait for the engine to rev to generate torque.

The solution I think for Trucks and other larger vehicles would be a series Hybrid set up similar to the upcoming Volt. Where in there would be a Diseal engine acting as a large generator to high capacity electric motor, keeping it charged at all times and providing torque. Locomotives have had this set up for years and it would just be a matter of reducing the size of the system and making it affordable.