It’s more “is it practical to access the cameras” than “can someone access the cameras”.

Look at it this way… How are “they” going to access your cameras? Are they going to have a mobile broadband connection for your vehicle, and upload realtime video data from multiple cameras in real time? Maybe for one car, but to do it for every car, they would end up choking out all the cell towers in an area. What about storing your data in the car and accessing it later? Well, storage is relatively cheap, but when you add in the compute required to encode that video, and depending on how much you want to store, it could easily be a few hundred dollars to include all that, and let’s face it, manufacturers want to cut costs, so who is going to pay for it? To top it off, if this repository of compute and storage existed, computer nerds would have found it and publicized it, so it seems unlikely to me that it exists for most cars. Finally, what data would “they” hope to get from real time-video of your car that is not already documented in other ways? The GPS unit can log your position and velocity with ease, your phone is an active listening device that follows you everywhere, so there are simpler and cheaper ways to get the same useful data as a realtime camera feed of the side of the road that don’t involve some huge conspiracy.