[Daryles]

"As far as I can tell, the circuit includes the bathroom light and fan, bedroom ceiling lights, reading lights, and the usb's."

Obviously this fuse runs several circuits as you have already figured out. For a fuse to blow instantly when you put it in means you have a dead short to ground on one of those supply wires.
Only proceed if you are experienced and comfortable working on LIVE circuits.
Get a multimeter.
Remove shore power.
Disconnect your battery negative cable.
You will need to pull the power panel to get to the back of it.
Take your time and take lots of pictures.
Then identify the output wires supplied by that fuse, remove them from the panel and separate them.
Take your multimeter and check each wire to ground (RV frame).
The one that reads 0 ohms to ground is the bad one.
To prove it, put all the wires back together into the panel except for the bad one.
Reconnect your battery cable. Put in a new fuse and check your circuit devices.
If good, now you have to figure out where the bad wire is shorted to ground and what devices it powers. This is the hard part...
You will need to physically trace that wire to see where it is shorted to ground. Probably a screw or staple through the wire.
In the IT wiring world we would use a wire toner or a TDR (Time domain reflectometer, expensive) to figure out where the wire is shorted to ground.