Every time we change oil in our class C they charge us extra

As others have already said, CHECK THE OWNERS MANUAL. If you have one it will be quicker to look it up yourself than to call someone else. If you don't have one it would be a great idea to get one for future reference for other types of repairs, maintenance etc. I suppose the next best option would be to call a Chevrolet TRUCK dealer service department and see what they will tell you.