Agree with snowbird on a poor connection being the most likely problem, and as was said most common culprit is a ground somewhere on the system.
As far as the converter, look on the website for your builder, most have at least some owners manuals available online. Hopefully you can verify appropriate size that way, or give them a call tomorrow, although 25 does sound a might small. Fridge even on propane still needs 12 volt electric available for the thermostat. Finding the open circuit should take care of that too.