You feel guilty for something you had nothing to do with?
In DRIVE or not, he should have set the parking brake.
There's far too may people with money out there, with no driving skills, buying motorhomes and endangering everyone around them. Everyone buying a large RV should have to demonstrate an ability to drive it before they're allowed into public with it.
Aren't you glad that this crash didn't occur on an interstate where you were slowed in traffic with him barreling down on you at full speed?
I hope for our sake, and all the other people on the road, that he and others like him give up driving.