So many times it is just a bad connection. I had the same thing happen, and found my negative line on the converter output terminal strip was so loose, no charge was going to the battery.
Of course, when you change out the converter, it all works fine again, but so often, just check the terminals right on the converter with your meter to see if it is getting 120 first, and if it has a good output second. You might save yourself a few hundred dollars and learn some good lessons.
Mike
2012 Winnebago Impulse Silver 26QP
2005 16.6 Double Eagle
2018 Jeep Wrangler JK
previously Snowbird Campers,
Triple E Motorhome and Fifth Wheel