Check the tongue frame just as it crosses the front of the cabin. It could bes stamped into the frame in a place protected from the weather.
It is also possible that the ID plate that was pop-riveted on 16 years ago has fallen off.
TT 2015 19RD "TheJayco"
TV 2003 F-350 "Montblanc" - Housebroken chore truck
Sitting in The Cheap Seats.
And proud of it!