I would search in Missouri, Kansas, Oklahoma. Farm country, lots of grain trucks, farm trucks, etc. I used to live in Missouri, and know there are many more junkyards and you see a lot more old trucks sitting in fields and beside barns than in Mississippi.
Agreed. We just drove thru Nebraska, Iowa, Missouri, Kansas, Oklahoma, etc. and saw lots of them. Like Patrick said, a lot of onesies in the farms, and usually the bigger trucks. Even up in Alaska, the larger trucks seem to be more common.