2010 Freightliner Columbia - Blower working but very little heat anywhere

Are you able to redirect the air flow to the various positions. Windshield, floor, dash ports.
There are electrical actuators under the dash that control the doors that redirect the air flow. On earlier models they were air operated, the anchor ends of the actuators were mounted to plastic that would break and you would lose control of the blend doors.
If the cabin air filter was that bad, I have seen the heater cores get plugged with debris. Maybe take the filter back out and try to get a look down stream towards the core.
Does the truck have heater hoses with shut off valves under the hood. Just follow the lines from the heater core to the engine and watch for a valve that maybe be fully or partially closed.

 

This video is for a Columbia.
(1) Freightliner HVAC blower motor. troubleshooting, diagnosis, repair - YouTube


1- is an air filter but there should be the one for outside air. The one you probably changed.
2 - is the heater core
3 - remove those screws to see what is behind there. Likely the blend door.
4 - this looks like a servo motor to operate the HVAC blend/direction door

Note the thumbs up/thanks icon near reply.

 

Columbia's have been gone for a while also. But if you take the panels off in front of the passenger seat is the blower motor not at the bottom? Take it out. Is the fan spinning on the shaft? Look in there At the core. Is it stopped up with corn dust, dog hair or rat stuff.
Wash and blow all that out as best you can while you are there. Even if that is not your problem. Then go looking for duct doors problems.