All those U-Hauls crossing the USA every day and who knows what they carry. No one checks ever on their contents as far as I know. What do you want to bet they carry all kinds of things besides what we think of as "typical" items to be moved? They could transport rocket launchers for distribution or relocation everywhere in the country. Also, dangerous chemicals, and no one would be the wiser. Including them for cursory inspection at our nation's weigh stations might nab us a terrorist or two and save a city as well.
The way things are going, even if LEO found dangerous chemicals [red mercury!] etc inside the U-Hauls, they would let the folks go anyhow as has already happened in Canada...and here with the "israelis."
Anyway, a private [non-commercial] trailer can not be searched without a warrant, even if the driver is arrested. A very private way to go, eh? And the jihadis know this I'm sure....