Installing the most Maxxair Vent Fan 4000K with the manual lift for the lid into our Ford E350, but the process is the same for any van. This is definitely something you can do yourself, and we've had no leaks since installing it, including a winter in western Washington.

Maxxair Vent Corp 4000K Maxxfan Plus Vent 14" 12V White $170
https://amzn.to/2XxfGPA

Butyl Tape
https://amzn.to/35BYFbf

Amazon Associates paid links