NCVT is a great idea and I keep meaning to get one. In the meantime, that little plug-in tester gets you a long way toward detecting problems before you plug in. Get one of those little "puck" adapters that lets you plug an ordinary extension cord into a 30A RV pedestal outlet. Put our plug-in tester in that puck and you can check the pedestal outlet before you connect. If it checks OK, two lights as in the pic above, then connect your RV, take the tester out of the puck and check your outlets inside.
It is very possible for some of the outlets on an RV pedestal to be wired properly and others NOT. The 20A GFCI outlet being wired OK is no confirmation that the 30A is also correct.