The safety switch keeps you from starting vehicle in Reverse or Forward. We can rule out the PCM and for now Ford chassis wiring as delivered to Thor. Somehow, somewhere it appears you have got power back feeding through the trailer side of your reverse lights (F34) into (F33) via splice S216. To verify this pull the F34, remove the reverse lamp relay and check for voltage at blade 5 port. You should have no voltage, but when you reinstall F34 you should have voltage which confirms the back feed. Put everything back except for fuse F34. Do a engine off reverse lights check just like you did before except you won't be pulling fuses or relays. At this point, if all is good, the troubleshooting will focus on the trailer portion only. This will involve crawling underneath the RV and tracking down connections between wire sections. As for that Red wire that is a mystery. Does that red wire terminate inside the wire loom going to the RV jack?
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