Hi Dave - I use one of those Chicom 12-24 volt (20 amp version) "converters", probably the same one you showed but they seem to change manufacturers' names at random...I use it to power the PRC-47 in the truck from the 12V batt and so far it works well, Lo and Hi power selected.  Another powers my PRC-174 in the house off the solar battery system.

I would suggest you run your test again but with a representative load on the converter as a better test.

FYI, Tim

N6CC