All you're gonna need is a No-Ground Plane antenna kit (Firestick makes good ones, as does Wilson). Motorhomes, boats, most 18-wheelers, they don't have metal bodies anymore, and No-Ground Plane antennas work well. You can mount it on the mirror, the bumper (tho the entire van seems like an awful lot that the signal is gonna have to pass through to get out), or on the roof. A really good spot, actually, would be on the vertical side wall above the driver's door on that slope.
Ah Turtle, the last time I checked, tractors were metal.
Motorhomes are close to what David has, Boats use water as a ground plane and pose a unique situation because there needs to be some conductor to make that connection of the water and the antenna system.
A ground plane is part of the antenna system to act like a reflector. Trucks use the cab/sleeper as a reflective surface - ground plane, Vans and cars use the roof.
Firestik seems to think that aluminum is not a reflective material, funny that all base antennas are made out of aluminum.
Motorhomes with a fiberglass roofs pose two problems, one is many have a rubber roof coating and the other is that that many lack a real metal structure to mount the antenna on it. I don't remember if it is fleetwood or another manufacture that uses no metal in the roof.
The antennas that they sell are not really that great in comparison to other configurations. They claim great gain but....
I did see someone take a short antenna, like a K40 and used a three foot square piece of 22 gauge metal plate mounted inside of the box and grounded to the chassis and the antenna was mounted to the metal plate through the roof giving enough ground plane surface area. The metal also reinforces the fiberglass roof so the stress does not crack the roof.