Many in the RV community use Dicor self-leveling lap sealant on around roof fans, AC units, solar wires etc… anywhere there is a potential leak or to repair an existing leak. It comes in white or black (and leveling or non self-leveling). Some say to use black only, even on a white van, as the white product gets dirty and yellows with age. I used it on my transit roof as a preventative measure to cover the the bolt hole roof caps that are on these vans.
2019 Transit 350 HR Eco Quigley 4X4