I have a 50amp/220V outlet in my garage at home. When I plug in at home, I use the adapter shown below. It utilizes only one of the two 220v hot wires. Given my 50amp circuit in the garage works off two 25amp circuit breakers in the house, it is easy to concluded that it provides 25amp service to my PC.
I hate to do it, but I have to challenge this; two 25amp 110V breakers ganged together will provide one 25amp 220V outlet, not 50amp. Irregardless, as you said, the adapter will provide 25amp 110V to the RV.
When using the adapter on a 50amp 220V service, you will have a 50amp 110V service. This would explain why gophoenix didn't trip the breaker when using the adapter. The power post at the RV site will have one 20amp breaker, one 30amp breaker and two 50amp breakers ganged together.
Actually, most RVs that use 50amp service are set up for two 50amp 110V main breakers in the rig, not 220V. That is why there are four prongs, two 110V hot, one neutral and one ground. You need the neutral to get the 110V service from each hot lead, if you wanted 220V, you would wire between the hot leads and not use the neutral. The 30amp plug has three prongs, one each hot, neutral and ground. The adapter uses one of the 50amp hot leads, neutral and ground. One 30amp 110V service equals 3,300 watts, two 50amp 110V services equals 11,000 watts of power; that is why the 50amp cord on those class As is so large.
Using the adapter presents a potential problem, the 30amp power cord from the RV is now attached to a 50amp breaker. If there is a short before the 30amp breaker in the RV then the 30amp power cord and wiring up to the RV breaker panel is not properly protected; however, if the short is on the RV side of the RV breaker panel then the 30amp RV breaker will do it's job.