My wife does not like for me to watch anybody back in unless they ask for help. (I don't always whisper as quiet as I think. In a restaurant, "Baby changing station in bathroom, wish they would change it to something quiet" has got me in trouble)
Over the years, most times a driver backed into something a 'Onback' was involved.
Once my TV was parked in my site, but close to the road to give some toddlers room to play. 5th wheel owner asked me to move it so he could park in site across the road. "Sure, but if you get to where TV is, you will not get in the site" He asked if I could help. His wife said she knew nothing about backing, (If you can't back a trailer, how can you tell somebody else how to do it?) Had her get behind the wheel, him walk next to me outside her door. Backed up the road, then pull back up to set up telling both what to look for in mirror.