I’m actually fairly involved in some of the issues surrounding this. 10 years tops. Too much money at stake. The only saving grace is that they might need human presence in the event of an emergency, but I doubt it. Your drone comment is probably accurate — much easier to have drone monitoring capabilities on the trucks rather than a single driver.
As for safety, it is pretty clear that the trucks only a few generations from current tech will actually be more safe than with human drivers. They will have fewer accidents but will have different causes than human driver based accidents. Congress will give an agency statutory authority to regulate the use of driverless trucks and vehicles, and the agency will make a safety evaluation that will clear the vehicles. Congress critters will have their hands clean because it was an agency that gave final permission.
My personnel opinion is that in 10 years America will still not be ready for driverless trucks, not even drone trucks. I don’t think anyone is going to be ready at that point to say a fuel truck carrying thousands of gallons of fuel will be okay to be driverless. Even if the technology is there, I don’t think people will be ready for it. And I don’t think the technology will be there yet. I think we will just be getting into drone planes making deliveries from Amazon on a regular basis in 10 years. Just my humble opinion.