The oil in Murphy Oil Soap is not derived from petroleum. That is why it is an excellent (benign) choice for roof membranes that have cleaning recommendations against using petroleum based products. There is a reason that Dawn is used to remove oil and grease - It is a petroleum based cleaner.
Use what you want. It is your RV, your investment, your call. I prefer to error on the side of caution and take the road that offers the least risk for harming the membrane. It is a significant structure on the unit.