Wrong assumption John. Both the fan-only and the AC functions recirculate interior air, the same fan for both operations. That's why there is a foam filter over each AC intake on the bottom of each unit. You should occasionally pop open the grill and remove, clean/wash, dry, and replace the filter (or buy replacements). The fan draws interior air in that grill, exposes it to any cooled AC heat-exhanger surface, and pushes it back into the living space, just like your furnace or AC in a house. Though they aren't exactly high-tech, keeping clean filters in place helps cut down larger air contaminants.
Like others, we also maximize 30 amp connections by using one unit on AC and the other fan-only in order to spread the cooler air through the coach more effectively.
Joel