I found a hidden vin number under my 2012 HTT. It is a huge white plaque, and totally invisible. If I recall the numbers are about 1" tall.
I found it underneath, I do have an open underbelly. As I recall it is secured to the floor, maybe the frame rail (???) near the the passenger and rear wall corner. I have to be laying in just the right spot looking straight up to see it! On my TT, when you find it, it looks to be mounted in a wide open location, and should be easily viewed from anywhere under the rear of the TT. But you just do not see it unless you are in the right spot!
Currently the TT is on winter vacation and is not home, and I wont be able to verify exactly were it is mounted, but I hope that helps.
If do locate another hidden vin, please post it's location, along with what year type of camper.
Reference material:
https://www.rvandplaya.com/vin-numbe...r-and-trailer/