Is my 2017 Trek Fuel EX 5 Stealth internally routed dropper seatpost compatible?
I googled the current Trek Fuel EX bikes a bit. 

The EX series feature internally routed shift and brake cables the down tube, with entry points near the head tube.

The EX 9 comes with internally routed dropper post, and seems to share the frame with the EX 5, so I'd say yes.