If the RV has an engine/transmission, we'll have the seller drive the vehicle while we pay attention to the drivability of the RV. But if it has an generator, we'll also test that. But it's recommended that the engine & transmission oils be tested to be very sure that there are no signs of burning, metals, etc present. It's not required, but is a good thing to do, especially if there is any suspicion of problems at all with the engines.