my 1994 Dodge Cummings Diesel 2500 runs cold (140 Deg)

What is your question?

Put in new thermostat no change. Radiator is full. After 12 mile drive the block was just barely warm, radiator was cool, heater put out just barely warm air, outside air was about 60 Deg. need to fix it soon we will head toward -45 in a few months.

How long has your 1994 Dodge Ram 2500 had this problem?

About a week