My 2000 Bigfoot uses silicone on the bond line, around the marker lights and the vertical hatches. On the roof they used a painted on product. We are only now starting the see areas where the silicone is starting to fail. Not bad for a southern ca outdoor kept camper.
When we talked to them at the new Bigfoot, they said decor for the roof silicone for vertical surfaces, but that it has to be very clean and dry for either to stick very well. When we replace our marker lights we will use silicone.
We use dicor on the roof, and it for us is lasting about two years for the dicor.