There's a number of ways Cory. You could use some of the MIG mud/dust products, but that's relatively expensive, or use the same stuff made yourself. Basically, a mixture of dry pastel dust, and a slurry mixed with the dust and Future, or even water.
The bottom section of the vehicle and around the road wheels and tracks can be done by building-up progressive layers of dry-brushed colours, and the dust etc on the horizontal surfaces could be done with pastels.
You could even mix real earth dust with water, and brush this on, for heavier 'dirt' areas; once dry, it will look like what it actually is - dried mud or heavy dust, depending on the application. The possible disadvantage of this method is that the finish can be disturbed and/or cracked with progressive handling, as it's not really possible to seal this without altering the texture and colour.