It sounds like the bike wedged under the motorhome and slightly lifted the front wheels off the ground. He said he couldn't steer either. I think not being able to use the front wheels to steer or brake would have significantly increased the stopping distance.
You guys are correct that in this type of incident, the driver in back is generally at fault--but there is an exception, and that is when another vehicle swerves in front of you. The idea behind this exception is common sense--you can't adjust your following distance when someone swerves in front of you and brakes hard.