I never really updated this thread after I finished the install.
One issue I had was trying to figure out where to mount the rear camera. If I mounted it on the roof, it wouldn't be able to see much behind the vehicle and if I mounted further back by the third brake lights, the flat base would never securely mount on the curved section of the van.
So here is the mounting solution I came up with, using the 3rd brake light for inspiration I had the following 3d printed:
Base and custom insert:
And all three together:
Next time I have the Interstate out I'll try to take a picture of it mounted on the roof.