2018 Pete 579 little to no heat

OK, first post here, looking for a little help. I'm in a 2018 Pete 579 and getting very little heat in the cab. The sleeper heat works fine, so I won't freeze or anything like that, but worried about the defroster not keeping up in the snow. So far, I've changed the heater controls, the blend door actuator and even taken the heater core out and cleaned it. And the water hoses are both hot coming out of the heater core. When I was changing the blend door actuator, I stuck a screwdriver in there, turned it all the way to the left, and got heat but not for long. It will give me heat for just a minute, then it cools off to just kinda luke warm. Nothing I've done has really helped much. I've got it all back together now, and out of ideas. So, is there anything I'm missing, or not thinking of?

 

Yes, it does get hot, but not for long. It goes back to luke warm fairly quickly. I did check the hoses when I started this little project, and the return hose was barely warm, even though the supply hose was hot. So, I thought i found the problem. Took the heater core out, blew air through it in reverse, then forward several times. Actually, got good air flow through it by the time I was done. Put it all back together, warmed the truck up again and both hoses were hot! So, I assumed that would have fixed the problem. But it didn't really help much at all.

 

Blower works fine. just blowing cold or luke warm air.

 

And you may very well be right! At this point, Im about out of ideas. I mean, there really isnt that much to a heating system, on any car or truck. Its just take air, blow it across a hot heater core, and presto, you have heat. But this one is kicking my butt!