Much harder than a manned one. You can secure unmanned trucks better than manned trucks. Truckers need to be able to open the truck which is a weakness currently. With autonomous vehicles you don't have that. Camera data allows you to see theft earlier (currently it sometimes happens while the trucker is sleeping in the front) and directly report it to the police. The video also provides evidence and helps the police identifying the exact vehicle and location.

I would compare it to banks. ATMs are still a target of theft but it's much harder to crack them compared to robbing a bank where tellers have access to cash. The same will be true for trucks. There will be some thefts that are more sophisticated (and successful) but the overall rate will go down.