Yup, you need to do some meter checks. Battery first with nothing running, battery with truck running, battery with shore power, converter output terminals.
My battery went dead and I found a loose negative wire at the converter terminals. Converter was running full blast trying to charge, but the loose wire was just heating up and creating a high resistance load.
If you can get to the positive and negative wires on the converter terminal strips, wiggle them and watch the 12 volt interior lights. If one is loose, the sudden connection will make the lights come bright.