Long story short, chipping is an overused weathering fad on models. Yes it happens on real vehicles, but usually ones that are poorly maintained. Most countries that make military vehicles paint them in a very durable finish. Some of the jeeps on Iwo were likely new, replacements for losses of the previous campaign(s). But many would have some service time as well. And rust on equipment is a no no in the US military. Clean it and paint it. In battle is one thing. Between battles is something else. 

As far as weathering goes, look at modern photos of Iwo Jima today to get an idea of the soil colors, then select your dust colors accordingly.

Maybe just go with some sand on the tires and lower body? The Iwo Jima sand is volcanic and looks almost black to me so I'm not sure what would be best for this. 

My sample of Iwo volcanic soil is a very dark gray, almost black. It's also kind of pebbley ( is that a word?) like very coarse sand. Maybe the soil at the north end of the island is a little different since its farther from the volcano crater.