That is not true at all.A 15/30/50 will power all outlets on a MH BUT its not enough power to effectivly run anything but the converter especially through 50 feet of cord. A 30 amp to 50 amp will also run all and any of your 120V requirements except trying to run 2 AC at the same time or any other combos that load to that degree
If when u pluged in your 15 amp cord and only had one powered outlet you have blown breakers on the inverter/converter system
Also when u clicked on converter you most likely turned it off that's why the batteries went dead