A 50 watt panel puts out a little more than 18 volts and 2.5 amps at max output, which is a little more than the parasitic draw of a typical RV. I agree with John Hiley, in that a 50 watt panel could be hooked directly to an RV's battery-bank, with little concern of overcharging - especially when the RV was being used. In storage, however, I'd want a controller to prevent overcharging, because parasitic loads would be the only load on the batteries.
When I installed our solar system, I completely bypassed the existing 10 watt Winnie solar setup - I just left it in place. After my system was installed, however, I noticed that the red LED on the One-Place panel, that indicated that the Winnie system was charging, was getting hot - really hot, so I disconnected one of the wires going to its circuit board. I think that circuit board was Winnie's low-end controller, or some type of current limiter - not sure.
Kev