REALLY blown 30 amp fuse?

Ok, so this morning, while we were hooked up to shore power, DH and I both heard this strange "rat-a-tatting" noise just outside the sleeping quarters. We didn't really get what it was. We tore down and got on the road.
When we reached our destination this evening, we plugged in to power ( or thought so) I didn't hear the "converter noise?" near the bathroom. I checked the battery test button and it was red. ( we are still hooked up to the truck)
DH checked the battery in the back, and the fuse was blown and melted both sides of the plastic. Any ideas as to what this is and why this happened? This is a new battery. We disconnected the battery from the system.
Any insight would be appreciated
Thanks, Liz K