My first thought is that your converter/charger is going bad. I say this from experience. A load test would show the battery condition and should be the first thing to do. Many auto parts store such as Autozone, NAPA and O'Reilly's will do the test gratis. My mechanic will too.
You can hook a test lamp or meter directly to the output of the converter and if it is going bad you will see the fluctuation there.
"I travel not to go anywhere, but to go. I travel for travel's sake. The great affair is to go". R. L. Stevenson
David Bishop
2002 Winnebago Adventurer 32V
2009 GMC Canyon
Roadmaster 5000
BrakeBuddy Classic II