RTGs are incredibly well built and designed to survive a launch accident (indeed, they have done so) without releasing radioactive material. A road accident would be nothing in comparison. As for theft, that seems very unlikely, the parts are well guarded and not just put on a truck and sent across country.
Nuclear missiles, fissile material, reactor cores, etc are transported on public roads - where else would they go? We haven't invented transporters yet so transportation is still roads, rail, sea, air - these things don't just magically appear at the launch site or at the power station. Someone needed to drive the semi that gets it there.
There is absolutely a chance of fissile materials being stolen in transport - that's why there's multiple levels of security and multilateral treaties with most countries around the world about this kind of issue.
