The standard test for the converter is to plug the RV into 120 volt outlet, Disconnect the negative battery lead from the battery, then measure voltage across the positive and negative leads AT the converter. No voltage means bad converter. Should get something like 13.5 volts. If you run this test with the battery connected to the RV then you may well read the battery, NOT the converter.
It would be my guess that your neighbor has correctly diagnosed your problem. For what it's worth, it's not likely that the issue was caused by trying the AC on the generator. Usually it will simply fail to power it and sometimes trip a breaker but it's not common for it to do any damage to the converter.
Good luck / Skip