I might be wrong about this but I got the impression the Zodi heater air tubes pretty much formed a closed system and it was only the burner which received and discharged outside air - the burner air being isolated from the air being heated by a heat exchanger.
So putting both the inlet and outlet tubes into the cabin is just going to recycle the air in the cabin while heating it in the process. The warmer temp will make the humidity less noticeable because less of it will be condensing but you won't be getting rid of it. You are just cirulating and heating the same air.
If the air in the cabin is more humid than the outside air I think you would do more to reduce cabin humidity by leaving the inlet air tube to draw outside air and routing only the discharge tube into the cabin. Of course from a heat transfer point of view this is not as efficient as recirculating the warming cabin air.
