I'm guilty of the same thing as most RV drivers.
I started towing a bigger, heavier rig than I ever had training for planning to learn 'on the road'.
That said, most of us do take it easy at first, try to learn and do get better over the miles.
As opposed to the professional truck drivers with whom we share the road. Who must undergo training, medical exams and detailed drivers license exams and road tests.
The biggest 'proof' I can offer that such OJT isn't inherently dangerous is insurance rates. I've found that liability insurance rates are not higher just because one is driving a motorhome/ towing a large trailer.
I'm absolutely certain that if there were high accident rates, the insurance companies would know and would charge noticeable higher rates.
Note - most companies which offer RV insurance will give a nice discount if a person takes an RV driving training session/ course.
As to type of accident/ accident rates - my RV insurance broker tells me that the vast majority of claims are for campground type damage, backing into something, driving under too low a tree and breaking something, failure to unhook water or electric, cutting a corner too close and hitting something.
Highway accidents are relatively rare.