I also have this issue on the truck I just got but my temp doesn't reach over 160 typically. I am a little concerned because its gonna get way colder up here in ND and I need working heat!

 

if your temp doesn't reach over 160, you need a new tstat...

my tstat keeps engine temp at 195*

if you want it warmer yet, you can get a 200* tstat for the 6.7L cummins

it is the same size and will fit in the 5.9L but it runs 10* hotter which means 10* warmer air out of the vents