Another simple question... is the fridge on? The inside light doesn't work until the fridge is turned on.
This feature is actually good because the light is disabled when you have the doors propped open to prevent mold when the fridge is off between trips.
On the other hand, it could be the bulb holder has rusted out. The one in my old TT rusted out around year 3.
2015 Rockwood Signature UltraLite 8282WS Platinum, GY Marathon LRD, TST 507RV TPMS
2005 GMC 2500HD CCSB D/A, Curt E16, Prodigy P2, Garmin RV760LMT w/BC-20 b/u cam