I have a Sportsmobile with a Fantastic fan and, tired of having to keep it shut in the rain, had a MAxxAir cover added. It looks good and I can keep the vent open when driving, also a plus. The air volume of this fan even on low is terrific, so, no problem with airflow.
The only problem I have is that if I don't use the van much in summertime, mildew forms inside of it. I've solved this problem by using truck washes and giving them special instructions to wash out the inside well. That's gotten rid of the mold but like everything else, has to be maintained. I do recommend them.