Anyone ever considered putting their ventilation fan in the floor rather than the ceiling to prevent leaks?
Many people do this, or upper & lower side vents located diagonally to create automatic, non powered or self powered ventilation. Any solution is better than putting holes in your roof, or forking out the bucks for those expensive roof vents.
Those of us with experience prefer window vans with many opening windows, problem solved with no van modifications needed. If more ventilation is desired, a cheap under $20 12v or battery operated fan will provide better ventilation than even 2 expensive roof vents, and without the leak problems associated with roof vents or any other holes in your roof.