Ok. As you suggest, I'll stick with my trusty multi-meter to check for bonding.
Now if I understand this correctly, your inverter's 120V output becomes bonded only when plugged into the RV. So I guess there's a connection between ground and neutral somewhere in the 120V distribution panel, or ... somewhere?
Not sure if Prog Ind would think that's a good idea (see quotes above).
Also this bonding doesn't seem to blow up the inverter (a good thing!) which implies that the inverter is not as cheap as it could be.
Think I'll email Prog Ind with a query about all this.