I thought JohnB had sent me a pic of one of these one time, but I guess I saw it somewhere else. I finally got the parts together today and fabbed one up myself.
I took a short 120v power splitter and put a 30A RV plug end on it. The plug cost me ~$8 at Lowes and the cord/splitter was sitting in the garage already.
Once I had the RV end on it, I plugged the voltmeter and polarity tester into the other end. Now, we can test voltage and polarity at a CG, before we risk damaging anything in our rig. Nothing special and very DIY. To me, it sure beats finding out about problems AFTER plugging the rig in!
EDIT: The inside of the plug is clearly marked, as to which wire goes on which terminal. Pretty much Gary-proof!