"Any polarity tester that works would be fine. BUT, what you really should have is a surge guard of some kind (TRC SurgeGuard, Progressive Industries, etc). It will test the polarity, voltage, and other critical factors before it allows power through to the coach -- and it will disconnect power if one of the critical factors goes bad while you are connected.
I carry a multi-meter and several different circuit analyzers, and have a digital line meter in the galley that will warn me of voltage, polarity, and other faults. But I would not consider being without my hard-wired SurgeGuard. It has disconnected power many times when voltage dropped or rose beyond safe limits. "
Search by zip code. Good results for a list of places, but no further information on each location.
Uses lemon juice and hydrogen peroxide instead of bleach.