That is a good idea.It's good that you're questioning this! I absolutely won't be welding accessories for my van without first disconnecting the battery. On past projects (Jeeps), I disconnected the battery, then touched the positive battery cable to the chassis for 30-60 seconds to discharge any stored energy, before welding. I'll double-check the BEMM first to see if Ford has specific recommendations.
For the awning brackets I'm currently fabricating, I bought and just received some 8mm threaded nylon rod, and will be temporarily attaching the brackets to the roof using the nylon rod/nuts to break conductivity (I'll leave the battery connected for this). Hein's PVC roof rack pads will break the current on the underside.
Craig
The Bemm makes welding on the van sound cataclysmic