is he ever spanning enough distance where he would stay at a hotel/motel?
does he sleep in the back of the truck?
does he own the truck, or return it to work and go back to a car in the day?
if he is staying somewhere, then trace credit cards (if he/they are dumb enough to use them), or names registered (again, if they are dumb enough to use the same).
if he/they sleep in the van, then ask about vans in areas overnight, patrol looking for them.
if he/they bring the van back to where they work... then I'm not sure - I would presume approach businesses to be wary of their employees.
I still maintain that the sniper is definitely NOT me.
Therefore if the police would simply arrest everyone that is not me, they will have their man.