Every extended service contract I've ever had for my RV and vehicles required advance authorization. There are procedures that the warranty company want the service provider to follow to assure that the part is really defective, hasn't been abused, has been properly maintained. Quite often, they want the defective parts retained in order to examine them to be certain they were really defective. It would be very easy for a service provider to defraud a warranty provider if checks and balances are not put in place.
It's really important to read the contract from time to time so you know how to obtain service in accordance with the agreement. And if you need any work done, read it again before you authorize the work.
When you had the work done, did you notify Camping World that you had an extended service plan that you wanted to use to get the work done? If so, shame on them. They deal with these plans all the time and should have known to verify that the authorization was in place. And it will be interesting to see their response to the failure to reply to your letters. You did mail them to the correct location - didn't you?