I have a 2004 Itaska 40wd and I have the same problem, and I located the relays behind the electric panel under the drivers seat, accessed by the outer door, I also checked the switch, swapped with another and tested the wires, but never did figure it out so I just disconnected all the rods so the doors would not stay locked, now I just use a key.
BTW: the original wiring diagram book is a bit vague on the location of the wires actually being run in the coach, I suspect that over time they have broken somewhere on their secret journey.
2004 Itaska - Horizon
IKS40WD 350 Cummins
2012 Jeep Wrangler