sounds like a great storyline.
proposal for map 3:
they couldnt find a working truck in map 2 and now have to walk highway surounded by woods and open fields with one or more bridges.
maybe they have to take shortcuts through those fileds/woods because of trucks blocking and reach the road again.
they could pass truckstops, farms and houses (but that may be too much for just one map)
just something like in "heaven can wait" ( can be found on l4dmaps . com)
hope to see your campaign