dust ingress coromal o/road caravan

G'day everyone,
I own a coromal PC511 o/road caravan fitted with a roof pressure hatch which is designed to stop dust ingress on dirt roads As I understand it you open the hatch prior to travelling on the dirt and the van becomes pressurised inside thus stopping dust entry when you hit the unmade road.
That's the theory but in practice for us it does not work as we always end up with a dusty interior.The dealer has been over the van and sealed any spots where dust ingress was indicated or looks likely.It would seem to me that the theory is somewhat flawed as surely if you had to reduce speed on a dirt road in a dusty environment[road trains] the pressure in the van would fall and upon acceleration dirty air would then flow into the van.

I wonder if any other coromal owners have experienced the same problem and could offer any advice ? Regards jonh y