Just some info about "rules of the road" out of the Colorado Drivers Handbook.
Merging: You must yield to all vehicles on that roadway. Do not merge if another vehicle must slow down for you.
Entering the Freeway: When entering a freeway from an on-ramp try to increase your speed to match that of the freeway traffic. Remember, you must yield the right of way to the traffic already on the freeway.
Freeway Driving: .....To be courteous to vehicles entering the freeway, you should change to the left (or center) lane, if you can do safely without slowing the traffic flow.
Following Too Closely (C.R.S. 42-4-1008(1)) "The driver of a motor vehicle shall not follow another vehicle more closely than is reasonable and prudent, having due regard for the speed of such vehicles and the traffic upon and the condition of the highway"
Out of the above, only the "Following Too Closely" is an actual law cited in the manual, the rest are rules. As I read it, the guy in the back gets the ticket, but unfortunately it very well could be a combination of fault between the two guys in front of him. The Motorhome may not have yielded forcing the freeway car to slow, or the freeway car may have voluntarily slowed to be courteous rather than changing lanes or doing nothing as suggested above. Without additional facts its hard to tell, but root cause could be very different than what we all think it is.
On a side note - I know following too close is inexcusable in any case, but how do you maintain enough distance in front of you driving a MH when any gap you create is filled within seconds by cars maneuvering through lanes to get ahead? I find it impossible sometimes.