30 + 20-amp pigtail cheater plug
I do not have any experience with this product. It would be an easy process to make one at home for a lot less than it is available commercially. Looking at it with an open mind I personally wouldn't have any use for it. In my opinion most newer Campgrounds already offer a full 50-amp service or a properly wired 30-amp service.

 Most but not all Campground Electric hookups have been fine for my needs. I did find a few low voltage and not properly wired places. I do check all Campground outlets with my homemade gizmo. (See it under "OUTLET TESTING" on the left)

A correctly wired 120/240-volt 50-amp service will give you 12,000 watts and a 120-volt 30-amp service 3,600 watts. A cheater pigtail if it works, theoretically would give you 6,000 watts.

Unfortunately this 6,000 watts is hard to get for a number of reasons. If the campground is wired correctly the 20-amp outlet will be on GFI so the pigtail doesn't work. Most older campgrounds with the 30-amp service will have the 20-amp and the 30-amp wires on the same Main breaker sharing the same HOT and Neutral wire.

Received the following FEEDBACK from abcarlson concerning the wiring and breakers used on some RV Campground pedestals.

It might be worth mentioning that on the 30-amp panel example, that even though there is a 30-amp outlet/breaker, and a 20-amp outlet/breaker, you may not be able to draw 50 amps. It depends on how the pedestal is wired. I have seen situations where the pedestal is breakered for 30 amps, or the 20-amp breaker is slaved from the 30-amp breaker.

The 50-amp situation is often the same, even though there is a 50-amp 240V breaker (100 amps 120V), a 30-amp 120V breaker and a 20-amp 120V breaker. The pedestal can be limited to 50 amps 240V.

It really depends on how they wired the RV Park. I've seen both situations: individual breaker per pedestal, daisy chained pedestals grouped together on a breaker. Either is legal, as long as the circuit breaker is sized to protect the wire downstream.

I would be concerned switching on appliances with an unknown amount of wattage available with this setup. As I understand it some people are happy with this but I believe an RV with heavy watts demands will have a problem.
I leave the technical discussion on this subject to others. If you would like to see some pro and con opinions also some not exactly correct information go to this LINK on RV.NET under Tech Issues.