90 E350 Heater blows hot air

I have a 90 Shasta Motorhome on a Ford \E350 Chassis. When the AC\Vent\etc. fan is turned on, it blows very hot air out from the back of the big black heater box at the passengers feet, under the dash.
It doesn't matter what temperature is selected, hot or cold. It always blows hot air, as soon as the engine is warm. I have looked around, but do not know if this is a vacuum control, or cable, and don't really know where they come in. It feels like the air is blowing out the back of the box, rather than from a vent. Is the box mounted on incorrectly? Anyone know anything about these controls? Thanks, I would appreciate any help.

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