First of all I would start by checking your battery or batteries if you have multiples.
checking them would mean using a battery hydrometer or taking them to a parts store to put a load on it or them.
If those check ok I would start troubleshooting the converter with a Digital volt ohm meter.
If you don't have one then its time to buy one and leave it in the camper.
I will post a trouble shooting guide on your parallax.
here is a great flow chart.