And there are a lot of trucks. On my trip west Ive noted that in places the trucks greatly out number the cars.I keep a couple spare gallons in the garage for this reason. My Mercedes uses a couple gallons every 10K miles so it isn't a big deal but it can be for the big diesels.
If they run out of DEF, there is going to be a lot of trucks, including semis parked. They cannot allow that to happen.