I wonder if the guage cluster is bad?
On my 06 (and most other cars sold in america) the temp guage is buffered from actual temps. They do that to keep people from "freaking out".
On my 06 it will reach it's "home position" in the middle at 170F and it'll stay there until 220F. Colder and warmer within that range **might** move the needle a hair either way. Can't really tell. But below and above that range and it clearly reads low / high.
Sounds to me like you need a scanguage. Use it to see what your trucks PCM is seeing. Which btw is a different sensor than the one the dash guage uses.