I rented a camping car and was later charged a cleaning fee.

Check to see whether the cleaning fee was originally included in the contract or if it was a compensation claim due to excessive dirtying of the vehicle during use. It is common for insurance to be included in the rental cost. But insurance may not apply if there is interior defacement or odor (smoke in a non-smoking vehicle, e.g.). Check the agreement first, then talk it out with the car rental company.