I've tried them all and found the BEST soap to be the ones sold at RV stores specifically for RV's. CW, every local camping store and RV dealerships that have an upfront little RV parts store, sell it
They have something in them that takes off the specific type of grime that usually adheres to our RV's and most have a wax in them that really makes them shine after just washing the RV. Use a real concentrated mix of it in a bucket of water and it takes off black streaks in a heartbeat too.
Full Timers RV Wash is the one I found to be the best. And in this case a specialized product is less expensive than the car wash ones.
http://mobilehomepartsstore.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=440140