The Foxwing can be attached either directly through the side of the vehicle (as on my truck) or to the crossbars on a roof rack. Putting holes in the side of your van may or may not appeal to you, plus it'll require backing plates and careful analysis of how well you can brace it, plus it may foul on the rear doors, plus if you have side windows, there won't (I don't believe) be enough space. And it's about ten times as difficult.
So . . . much better, I think, to mount it off the roof bars holding up the Airtop tent, and with any luck, you've got Yakima, Thule or RhinoRack crossbars, as fitting kits are made for them and make it a relatively easy matter. Another brand of crossbars might also be work to work, but it's not clear. Check out the installation instructions here:
http://www.rhinorack.com.au/rhinoweb/rhinodata/instructions/accessories/31100_foxwing.pdf
if you haven't seen them already to see if your crossbars can be made to work.
If you can use one of the Foxwing fitting kits, then there's the issue of whether the crossbar separation can be made big enough (probably) and whether a medium Autotop is wider than the crossbars you have (seems unlikely).
The Foxwing is a fine piece of gear, so best of luck on making it work.