Get a good EMS from someone like Progressive Industries. We have
this one hardwired in the compartment where we store the power cord.
Plug it in, the box does it's tests and connects in less than 30 seconds with an audible click. You can watch it cycle through each leg and see the voltage and the draw from your rig.
Ours has found at least three mis-wired posts that I recall over the last 7 years, also has tripped for power surges or low voltage.
Last year we had a neighbor lost his flat screen TV and had other damage simply because the 30 amp post was worn and caused his power cord to overheat and short out. An EMS would have saved them from the damage.