Welcome to the forum....and the dark side

When you replaced the multifunction switch did you take a look at the connectors on it? The wires pull out of the hardshells quite regularly, especially if it has tilt.
These trucks do suffer sometimes from water getting into the under dash fuse box and GEM (Generic Electronic Module) which is attached to the back of the fuse box. Usually the water gets in there from a leaking windshield. Your symptoms aren't common for this problem but anything is possible. The common symptoms for that problem are the heater won't turn off with the key out and things like that.
You said you checked the trailer wiring. Have you tried removing the trailer harness from the system and see if that changes anything? I've been fooled by bad trailer harnesses and plugs before.