If a mechanic changes your oil, prices will be the highest. A gas RV with a Trition engine usually will cost around $200 dollars for an oil and filter change. A diesel motorhome with a standard Cummins or cat will usually cost around $600 to have oil and filters changed. Part of the big price difference is the significant quantity of oil diesel engines take. These prices are with standard oil. However, synthetic could cost twice as much.