This is the diagram for your trucks HVAC system heater control.
This is the diagnostic tree from the service repair book for your Dodge ram.
This is the duct flow air chart for your HVAC box that is used in your ram truck.
You have told me that all of your mode doors work ok , so that leads me to think that the operation of your blend air door is the root cause of your heater malfunction. Use the wiring chart I have provided to test for voltages and grounds, If its not in your electrical wiring , and your switch is working correctly, the chances are , either your blend air servo motor is binding up or is defective, or your door in your HVAC box may be bound up or broken. Possibly some foreign matter has gotten into your air box and bound it up or the door hinge point may have snapped. --------------------
This is where your truck taps into the engine to supply your HVAC mode switch with the needed engine vacuum to operate the other none electrical servos in your HVAC box.
Under your windshield cowling, is the location of your vacuum reservoir, to supply vacuum even when your engine is under heavy load and producing low manifold vacuum. But your mode doors are working fine, so I think this part of your system is not the cause of your problem.
The Possible causes listed in the factory manual for heater outlet air temperature being unadjustable are as follows.
1. A faulty heater-A/C control.
2. A faulty blend-air door servo.
3. A faulty, obstructed or improperly installed blend door.
4. An obstructed cowl air intake.
5. The engine cooling system.
This is the exact info I have available to me on your Dodge ram , I hope this helps you to track down the source of your problem.
Please let me know if I can help you in any way to further diagnose this problem with your vehicle.