Open Ground when on PSW Inverter -- Experts please advise

I installed a small solar system on our TT last spring. It worked fine last summer. Now to round out the system, I just completed installing two golf cart batteries, and an inverter, a Morningstar SureSine 300 watt. The output from the inverter is wired to a 30 amp RV style female receptacle. So to feed the outlets in the TT, I plug the shore cord into that receptacle and turn on the inverter (this inverter has connections to wire a remote switch to, which I did). The green contact on that receptacle is grounded to the TT frame.

Everything was all hooked up, so I fired it up to test it. Then I used a 110V household receptacle tester, the kind with 3 lights, various combinations of the lights showing correct, or specifying the nature of a wiring problem. The receptacle tester a single light indicating "Open Ground". I have checked all my wiring and my connections, and don't see anything done incorrectly, per the instructions. All connections seem tight.

When I plug into shore power, the idiot light tester lights up indicating everything is correct. Is this normal or a bad sign when plugged into the inverter? If it is a problem, how would anyone suggest I troubleshoot it from here?

Thanks in advance. I have to leave right now for a short appointment, will probably be absent for an hour, so I apologize for a slight delay that may ensue in answering any responses.