02 Cummins heat

[FDCaptA110]

I recently purchased an 02 2500 Cummins 4x4 (very clean and sorted btw). We're having a pretty good cold snap here in Southern Ontario...while driving, the temp is usually right around 90⁰C on the gauge. The heat coming out of the floor and defrost vents is luke-warm at best with the temp dial maxed. I've read all the info on the blend door coupling...so I got looking, and it appears the the plastic coupler has been updated with a metal one. The actuator seems to operate when the temp dial is turned. When touching the bottom of the heat box where the core sits, it feels warm but not overly.
Other than a plugged core...what else could it be, or what should I check? Is it possible the blend door and actuator are out of sinc?
Thanks!