Powering laptop from 12V sources without inverter
Yes, tons of power is wasted going from 12V to 110V, especially when all you do is to stick it into a psu which also loses some power turning it back into low voltage DC.

You can buy a DC/DC converter which will deliver a 9-20 V DC adjustable voltage when given 10-24 V DC input.

I've built a SEPIC style converter before, for just this sort of thing:
http://dren.dk/carpower.html