The plot thickens....
Went out there today and the mystery fuse is blown. No lights in the back. But that's not the circuit they're supposedly on.
The "spare" is hand-written on the box. I'm thinking it's not really a spare. But there are 2 circuits involved in illuminating those back lights for some reason.
I've had the rig since December of last year, plugged in pretty much continuously. Never had this problem. I'm thinking it's the shower fan. I recently replaced the motor and fan blade. But the fan blade spun the wrong way. I reversed polarity on the motor. But the switch is bad and runs at random. I'm thinking there's some resistance through the switch. Gonna clip the wires and see.
But I'm out of 20A fuses for now....