Pop top or hard top ?????
Pop Top and even hard top yoiu have to make sure your not going to cut through a roof frame, it doesn't take much for the pop tops to bow and running cable inside them is near impossible, braces and styro insulation........... a fibreglass roof is really a sandwich, styro stuck between 2 sheet of ply..... so make certain of everything before you cut a hole.....
The thing with these pressure fan things is you have to go around the whole van and seal all the gaps, you will find dust will enter around the fridge, in through cable runs , door vents and water line entries.... On my old 14.44 I had to seal where cables, plumbing entered, ran foam door seal around the fridge, even filled huge opening where cables ran into a cupboard, still got small amount of dust and found it was entering where the tail light cbales came into the van buried in a cupboard and wall............. no dust, Big Mal required less work but fridge edge needed doing and once I fitted a flap a drop over the door vent, no dust............
In all cases you must have your drains closed, blocked with plug or whatever, this is often forgotten, drain on side of van will create a low pressure in van from sucking the air from an open sink.....
Never block the fridge vents, ever, the back of the fridge should be isolated from the inside of van anyway...driving a long distance with them closed you may as well turn your fridge off................. a bit of dust in there does less harm than blocking it off for hours... fridge be working its guts out and still turn into a food warmer.