Open Ground when on PSW Inverter -- Experts please advise

Incorrect. The tester works just fine. What user may not realise is that the ground prong of these inverter's outlet has voltage with respect to ground. This defies the purpose of the ground prong which is suppose to connect the outer shell of the appliance to the ground potential to prevent electrocution. So if your RV happens to be connected to shore power thru shore power cable and you use inverter you might be electrocuted. It is best to avoid inverters like that especially when better inverters are available.