Semi truck drivers are brilliant at judging distances - their job entails parking massive trailers with inches to spare on either side and negotiating through heavy traffic. I'm sure the semi driver realized there was not enough space for the Class C to pass and assumed that Mr Speedy RV would exercise a shred of patience and back off a bit?
Many trucking companies heavily penalize drivers for having an accident - even a not-at-fault one. I really doubt the semi driver would risk that - his only fault was not realizing the stupidity/impatience of the RV driver.
Also - the RV driver was super-lucky that his rig ended up sliding backside-forward when it peeled open like a sardine can and burped a kid out the front of the cabover.