There is a current limiter right next to the junction box that the umbilical goes into. That protects the battery from a short caused by cutting the umbilical from a variety of causes.
Sometimes plugging in or unplugging with the truck running (and the battery is low) can cause a surge that can trip that limiter open. You won't know this has happened when you are connected to shore power as the converter will provide all the 12 volt juice you need.
There is a little button on the short side of the limiter that can pop out and "open" the limiter. Just push it back in.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW