We have a 1991 Sportsmobile (Dodge Ram van) and we have a minor leak in the penthouse. It only leaked a little last winter when it was very rainy and windy, and the van was sitting out in the weather (not being driven). One corner of the canvas got wet, and the carpet in the "basement" (tool storage area under the floor) got wet. The penthouse roof has some hairline cracks and some algae or fungal discoloration (spots) -- see photos below. The Sportsmobile service representative I talked to said the hairline cracks are probably superficial and the leak is probably coming in via the rubber gasket at the base of the canvas, not through the fiberglass roof itself.
To seal up the gasket at the base of the canvas, the Sportsmobile rep recommended 100% silicone sealant ("NOT caulk" he said). But I've read elsewhere that butyl seals much better against metal roofs and rubber gaskets. Does anyone have a specific product recommendation for sealing the rubber gasket against the metal van body?