Maybe explained already. Didn't read 3 pages, but yes the OPs experience is valid.
Cold air sinks, heat rises, opens roof vents and/or a fan and you're exhausting hot air and lowering the air pressure the fan on the AC is fighting to blow air into the camper.
Assume an air tight space. Pretty soon the space is going to pressurize to the same pressure the the blower is capable of producing and then no more air flow. If the camper is closed up then you get less air flow.
That's why re circ on the car cents blow harder. Not trying to pressurize the cabin with "new" air. Same effect by rolling down a window in normal mode.
