I have driven RRB for Virgin and Northern and was told to have nothing to do with tickets - and rightly so as I would not have had the knowledge to do so.
However, most passengers boarding the coach would wave a ticket at the driver and my guess is that the number actually travelling without a ticket would be pretty small? I wouldn't have thought it was worth the hassle of having sufficient staff for the TOC's to check.
I can say that RRB passengers were deffo the most grumpy set of passengers I carried and who can blame them, having been shoved off a train service onto a coach which was going to take a lot longer?
As an aside we were not allowed to drop passengers anywhere except the station which used to upset a few heading for Kendal as an example, as we drove straight through Kendal to the official stop at Oxenholme. If you broke the rules you got put off the job.
I've also driven them in various parts of the country. Agree that we don't usually get involved in tickets, though on the late-night ones asking to see a ticket is one way of keeping the drunken blokes off.
Regarding stopping, if people ask me to stop early, I usually say that we can only stop if someone feels ill and needs air. I'm not one for driving straight past where people want to be just to be a jobsworth.
I once had an FCC RPI accompany me on a journey. I think she was off-duty but still wanted to check/sell tickets.
I've also had TPE staff count passengers onto my coach, selling tickets as needed.
One ticket seller had a passenger offer two tenners (for a £12 fare) say to the drunken guy that he hasn't got any change but will let him travel for £10.... Guess what no ticket, and tenner in his own pocket.