I started out in a tin top 1961 VW camper van, opening windows all around, and no roof vents. It was an oldie, but it was a factory westy with the beautiful varnished birch interior. It did have a house battery, charged while driving, but the only power in the back was a single cigarette lighter outlet, and a single fancy dome light. The stove/heater, fridge, hot water, and oscillating fan, and the 2 cabin lamps were all kerosene powered, no propane on board. Behind the drivers seat was a single seat facing backwards for the dinette which had a portable toilet under the seat cushion. It had bucket seats up front, so you could walk through to the back, and the tiny but efficient kitchen resided behind the passenger seat. It was designed so you could use the kitchen from either inside or outside. It had a fold down table, a lot of fold up/down counters & shelves. The double bed converted into a front facing sofa for the back side of the dinette and for lounging. The only thing missing was the original portable kerosene powered swamp cooler that the seller kept, although he did show it to me, and I was later able to replace it from a wrecking yard. Even without the swamp cooler, with opening windows and a fan, combined with parking in the shade, I was pretty comfortable. Equally important I had no leaks, and no moisture or mold problems.
Later I had other camper vans and motorhomes, and sooner or later the roof vents always leaked. Of course it was raining when they leaked, so you couldn’t immediately fix them. The misery of roof vents was very real and very frustrating. So I have grown to hate them, or anything else that requires putting holes in your roof.
If your back or side doors have windows, you might be able to replace them with opening windows from a wrecking yard.
When I had a cargo van, I wasn’t happy with 2 expensive fantastic roof vents with fans, and wound up building a folding box out of coroplast (plastic cardboard), to fit in a front window, with a 10" O2Cool ( under $20 at Walmart ) facing towards the rear. With the other side front window cracked, it did a good job of circulating air through the whole van, much better than the 2 expensive roof vents. Plain cardboard and duct tape can work if funds are tight.
The main people pushing roof vents are getting paid for it. There are many better and cheaper choices that don’t involve putting holes in your roof. After all is said and done, if heat is your problem, shade will do you far more good than any amount of insulation. You can secure screens to your front windows with magnets. Those automatic closing patio door screens work well if you are someplace you can have your side and/or back doors open too. Screens tend to keep much of the rain out too as long as it isn’t a wind driven downpour.
"Stay home, stay safe, and remember social distancing." ~ Van_Dweller