As a troubleshooting aid, turn off every 12v switch that you can find. If the fuse still blows when you change it, the short is on the supply side of those those switches, not on the load side of any one. That helps only because it reduces the possible points of failure by about a third.
Your next problem is to figure out how all those circuits are tied together. You might have 3 or 4 wires coming out of your fusebox of the same color or there could be just one wire that is spliced somewhere into 2 or 3 routes, or all those devices could be daisy-chained, one after the other. You can bet that Thor elected to use whatever method required the least copper.
A wiring diagram would be a huge help. You should be able to download one here:
https://www.thormotorcoach.com/owner-resource/
I like your theory about the slides catching on something, if the lights were working before you made camp. Have you tried retracting the slides to see if the short goes away? You're likely to burn a fistful of those little blue fuses before you pinpoint this issue.
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