In my vans, before this one, all of my roof vents leaked! Never did try to fix them though! This one I'm in now, does not have one, and it has a conversion van ceiling! So I'm still not sure about cutting open my roof!
On staying cool, I park in the shade, also have roof racks, with a so called curtain rod attached ! I got two large umbrella canopies & cover the top of the van when needed!
On the inside, I have a portable van, and recharge the battery daily! Also cover up all the windows, so the sun won't bring the heat in!
A vent does work, as heat raises, and the vent lets it out, more so, when there's a vent near the floor!