Is there a mechanic on duty there

Not 100% sure but highly doubt there is an RV mechanic or RV repair service provided by Fort Wilderness.

For what?

Can't answer.

I saw a a/c guy from outside come in and work on camper

No and I'm pretty sure they have a policy about having RVs/Vehicles not being worked on while staying there. You may be able to get with the front desk and possibly have an exception made depending on the situation. An outside company will need to be contacted for repairs.