There is space in most trucks. My Jeep Wrangler has enough room to stick two 1-gallon water bottles in the engine compartment -- one on either side -- and actually be drivable that way.

I prefer smaller bottles, either .5L or .75L. I can stick one or two on the dash to defrost, a couple more in the engine compartment if I want. If I run out of gas I can shove one under my coat to defrost using body heat. Smaller is better with the last method.