GMC Repair: no heat, coolant level, gmc sierra

Question
I recently purchased a 97 gmc sierra k3500 w/ a 6.5 L turbo-diesel.  After a few days running it in about 20 degree weather, i noticed the heater refused to blow hot air. I've heard that diesels just inherently don't get as hot as a gasser, something about them being more efficient. Could this be a problem with the heater core, or maybe one of the thermostats(2 on this model engine) are stuck open. My temp guage doesn't move above 160, no matter how long i been running it. Assuming the guage isn't malfunctioning, doesn't that basically rule out the heater core? If so, what are the chances of both t-stats being stuck??? I live in northern alaska where winters are like 40 below for months on end. Not here yet, but we just had our first snow so it's kinda crucial.
thanks,
antonio

Answer
Hi
I would check the basics first, proper coolant level, etc...
There is a possibality the 'stat(s) could be stuck opened, or maybe missing.
I would really concentrate on the stats first. Make sure there are no leaks. Make sure there are no air pockets.
I think you're going in the right direction.
Please follow post back, if needed.
Pawl