Let me add something to what aardvark said.
Ideally, you would plug that tester into the park’s power supply BEFORE hooking up. If you have a 30 amp plug – which is different from the normal household plugs – you would need a 15 amp female to 30 amp male adapter. You would plug the tester into the adapter, then plug the adapter into the 30 amp outlet in the park. Once you’ve established that the outlet is wired correctly, no reversed hot and ground or anything like that, go ahead and plug in.
(Campground “handymen” often do the maintenance on these things instead of licensed electricians, and they don’t always get it right!)
Once you are plugged into the shore power, I would plug that tester into one of your inside outlets so you can keep an eye on the voltage. On a hot day, when all the RVs are trying to run their air conditioning at the same time, the voltage can drop, causing a local “brownout”. If the power falls below 110 volts, it can damage your air conditioner if you keep trying to run it. By damage, I don’t necessarily mean it will burst into flames or fail, but rather you can wind up shortening it’s life drastically.
I don't like to make plans. They cause the word "PREMEDITATED" to get used in court!
DON'T FEED THE VULTURES!
My Body is a Temple! Ancient, Crumbling, Probably Cursed . . .