Just remembered another tool or two.
A Three Light outlet tester and a 15-30-50 adapter (that is two adapters the 30 in the middle is common to both) so you can check the park's polarity and ground, A Volt meter such as the Kill-a-watt that plugs in is handy too.
You plug the 3LT into the K-A-w, plug it into the 15-30 PUCK style adapter, plug that into the 30 amp outlet on a 30 site and if the lights are not right or the voltage on the meter (press volts) park elsewhere.
On a 50 you add a dogbone to the chain.. This leaves one lead untested, but odds are if they got 3 right #4 is as well since there is no other option.