I concur with your thought. My understanding is that the filter is directly before the evaporator coil. So all airflow goes through the filter to protect the coil, regardless of whether in recirc or fresh.
I’m not 100% sure on this Ram because I haven’t needed to change the filter yet. But on every other vehicle I’ve changed them on, the filter was always right before the evaporator coil. Would be silly not to protect the coil during recirc as well, since there is still dust and pollens that get into the cab every time the doors are opened.
Edit: I just went out and checked the truck, figured might be good to check the filter since I’m at ~22K miles. The filter was not too bad (see pic), but confirmed that the filter is directly upstream of the evaporator and blower motor. Blend door to switch between fresh and recirc is above the filter. So all air goes through filter, regardless of blend door position.
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