From what I've seen, I think Tiffin has been pretty consistent in putting the switch/outlet in the front right cabinet to the right of the TV and facing the co-pilot seat (i.e. upper right corner of windshield as you look out of your rig). If it's not there, I'd just unscrew those panels to see if somebody rewired it. Normally the TV antenna input goes into that area and after being amplified gets split into the coax cables that distribute the signal to each of the TVs.
If you still can't find it, call Tiffin tomorrow and they should be able to tell you if your rig has the amplifier/switch in a different location.
2016 Tiffin Phaeton 36GH
2013 Nissan Xterra S 4WD manual transmission