When you said, "new Forester on a Chevy Express chassis" I assume you mean a 2015 Express? I had a GMC Savanna and did work on the dashboard. The first step is to remove the kick panels on both sides. The whole dash plastic surround is just held in with clips, as I remember. Just start prying and pulling it out of the clips that hold it in place. It can be removed with the steering wheel in place. This fellow removed his to replace the radio, but you can see the plastic removed:
From a Chevy Express to a DIY Customized Camper Van: Replacing the Factory Radio
If you've got the space, I'd think about mounting a couple of 'under dash' power outlets in the passenger knee well or on the back or sides of the motor cover as an alternative. One or two of the outlets could be wired to your house batteries so you don't need the ignition on accessory to power the outlets when parked.