It's fairly easy to test the converter. While plugged into 120 volt power, remove the negative battery lead then measure voltage across the positive and negative leads AT the converter. No voltage means no converter power. If you do not disconnect the battery lead you will be measuring the battery so be sure you disconnect it.
Of course there is an EASIER way to get less pricises tests and that is to plug into 120 volt power, disconnect the battery and see if 12 volt items will work (water pump, lights, bathroom fan etc).
2011 F-150 HD Ecoboost 3.5 V6. 2550 payload, 17,100 GCVWR -
2004 F-150 HD (Traded after 80,000 towing miles)
2007 Rockwood 8314SS 34' travel trailer
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