I have just had a similar problem rectified, after struggling for about a month to try and resolve it.
It starter with the 30A fuse (that is between the battery and junction box) blowing when we tried to camp without power.
When we got back on power, everything worked.
Then eventually nothing worked, plugged in or not.
We tugged, poked, prodded, wrapped things in electrical tape, took things apart, put them back together, priced out converters.... tried it all!
Finally took it to service department as some troubleshooting suggested by a member here, pointed to a short in the 4' section of wire we couldn't see - from junction box to disconnect switch, it runs in the floor void.
In the end, that was the issue! The wire was pinched between floor and frame. For seven years (I bought this new in 2010) it has been wearing away and just now causing issues.
So glad we solved it but it'd sure be nice if they were just built properly to start with.