I was just into the office of the only KOA in Canon City and asked about what you were referring to. They do not charge extra for arriving late, in fact I was told they do not charge for arriving early either. There is no problem coming in after dark either. I nor the KOA folks have no idea where you got the info.
The folks at this KOA are the nicest folks you could find. I normally do not respond to these kind of post but seeing how I was at the KOA I thought I would find out.
I would look at the link for when you will be able to pass through on your way to Canon City and plan accordingly. Feel free to give the KOA a call to double check my info 719-275-6116