Greetings & Welcome!
The unfortunate truth is that cargo vans make terrible campers… Ventilation is a very important factor in campers, and even roof vents are extremely inferior to plain old opening windows. The promoters promote cargo vans for THEIR best interest, not yours, because they cost considerably more to convert, which potentially means more of your money in their pockets. Never mind the fact that they’re setting people up for misery & failure.
To be successful, you need ventilation that will circulate throughout the whole van, or you’ll be fighting moisture/mold issues. Step vans solve this problem by having fresh air vents near the floor in the front, and on both sides at the rear just below the roof line. That system works quite well, and due to natural convection (heat rises), it requires no fans to make it work, although fans can be added to increase efficiency if desired.
Cracking your front windows isn’t nearly as effective as foot well vents, unless your rear vents are located near the floor, but then you have exhaust fumes danger. To compound this, most modern vehicles don’t have true fresh air vents, they go through the heat/air conditioning system, and are never free flow fresh air vents.
Roof vents, or any holes in your roof, are a very poor choice… Floor vents are an equally poor choice, so opening windows remains the best possible choice.
The people who succeed at this lifestyle usually do one of two things, most will just switch to a window van or bus, or some will add opening windows to their cargo vans.
On another note, the necessary ventilation, totally defeats any insulation, so don’t waste your time or money on that. I have a high top window van with windows all the way around, and no added insulation or roof vents. I am also an extreme weather camper doing winters as low as -60°f and up to 120°f+ summers. I rarely cover my windows, yet have no problem maintaining comfortable temperatures inside both summer & winter. Your comfort depends on your heating, cooling, & ventilation systems, not insulation or covering your windows.
My first choice would be to cut my losses and switch to a window van with many opening windows before even starting. That step alone will save you a ton of money, both now and in the future. With a passenger van, the floor, walls, and ceiling are already pre-finished, and that alone will save you a ton of time, work, & money. Then you can merely remove the rear seats, and move your new camping interior in and secure it. I don’t drill a single hole in any of my vehicles because I’ve learned better choices.
My second choice would be to add opening windows in your back doors, and add rain guards to your front windows so they can be cracked while still keeping the rain out. Then a cheap fan blowing outwards in a back window will draw air in through front windows and through the entire van and out the back. In colder weather, just having both front & rear windows cracked, without a fan, should be sufficient. You might need to create rain guards for your back windows, depending on window type.
A friend in a Transit Connect added sliding windows all the way around his, and went to a place with a 3D printer and had clear plastic louvers made for all of his windows so he could leave them open in the rain, and they wouldn’t obstruct his view. He’s a full timer, and very happy with his decision on the windows. Prior to the addition of the windows he was miserable with a lack of a view and lack of ventilation. He’s in Seattle (rainy & high humidity) and the windows totally solved his moisture/mold problems.
Good luck, & keep us posted.
"Fuel powered everything for the win, preferably hydrogen." ~ JustTheFacts