TxBriley - excellent question, as I had the same one before my first fuel fill-up!
On my 2015 Platinum, it was on the floor held in place by a clip in the bay underneath rear seats directly behind the driver's seat (left rear passenger)
On my 2018 Platinum, it is tucked onto a fabric seam behind the right rear passenger seat
= You need to pull the top half of the right rear passenger seat forward
= Factory funnel is tucked about half-way up the frame by the door jamb
This is a better spot as it doesn't take up what little space is available in the old spot on my 2015.
Full disclosure: I cannot take full credit for this, as my Service Manager pointed the new location out to me.
2018 Ford 150 PS Diesel Platinum FX4 in Ingot Silver w/BuiltRight Rear Seat Release, Gatorback F150 Splash Guards, Lloyd Custom F150 Luxe Carpet Mats & Titan 40 gal replacement fuel tank
FORScan mods: Auto Start/Stop Disabled, Bambi mode, Digital Temps Above Gauges, DPF Full %, OCR (Operator Commanded Regen) and Heated & Cooled seats on Home screen