Easily made for $2 or $3, you don't need to buy the expensive one from Progressive Industries.
Go to any big box hardware store and buy a 3 prong plug. Take a short piece of wire, 1" to 2" long, and strip both ends. Put those stripped ends under the screws for the NEUTRAL prong and the GROUND prong.
DO NOT PUT ANYTHING ON THE HOT PRONG!. You are done. This does have to be plugged into the generator. Putting it somewhere in the RV won't work. (Of course, put the cover back over the wire. This picture left the cover off for clarity.)