On my TM there is a "wheel" next to my lights. I thumb it to brighten, or dim my dash lights. Assuming you have the same, and have checked it and it is not working, then I would replace the dimming wheel/switch.
07 TM 40D;ISL 450 Cummins(upgraded). Very happy w/coach
Gnarly toad to scoot around.
Geocaching, hiking, fishing, National Parks, Civil War sites.