One of the Suburban manuals talks about a "limit switch" that is temperature actuated to prevent over-heating and a "sail switch" that ensures that there's a correct amount of air flow. The NT-20SE manual has both of these indicated in it's circuit diagram.
My guess is that the system is just not working efficiently. It's probably not dangerous, or the limit switch would cut in. I suspect that that the outside exhaust stack is removing a lot of heat that could be kept inside if there was more inside airflow, and if there was less re-cycling of heated air.
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