Dave:
I'm pretty sure that there is no vent in the stock A/C, and what you are feeling is the recirculation of the warm air at the top of the motorhome. One problem I had (and still have) is keeping the thermostat too low in the low fan position, which results in the evaporator freezing. I haven't found the ideal mix of fan speed and thermostat setting yet. I did install a fantastic fan in my unit when I bought it, which gives pretty good circulation inside any time the temp is lower than about 80 degrees. I see my clients in my unit, and use that fan to minimize the use of the generator during the days.