geO,
This is interesting. On my 2015, all the windows, maxxair vent, bathroom fan, plumbing vent, and all the other penetrations were factory sealed with 3M Fast Cure 4000 UV. Oliver uses this all over the trailer inside and out. The porch lights and the markers were factory sealed with silicone. Many of those sealed with the silicone have failed. I would not use silicone over the 3M 4000. I had a few gaps here and there in the window trim that I redid. And I also cleaned up some "less than adequate" joints on the interior, especially in the bathroom and around the kitchen sink. The only issue with the 3M 4000, other than it's cost, is that once opened, it has a very short shelf life. So get all your spots lined up and do the inside and outside all at once. Cut the tip on your tube sparingly! Clean your rig well to remove any dirt and wax. Give a quick pre-wipe with acetone or MEK (my preference) on a rag around where the joint is. On the inside, I then tape both sides of the joint with thin blue tape (delicate surfaces). Carefully squeeze a fine bead in the joint and smooth out with your finger. Immediately pull the tape and lightly swipe again to get the ridges from the tape to blend in. You may not like to, but I wipe a little paint thinner on my finger to lubricate it a bit, works good. Too much sealant in the joint and you'll have a mess. Use the sealant sparingly. The first wipe should be tight to the tape edge, not going over the top. It's a judgement call depending on the size of the fill. The 3M 4000 can clean up with paint thinner so if there is some that gets away from you, you can carefully clean up with that. The stuff skins over pretty fast so be organized, work small areas, and don't forget to breathe! On the outside, I just very carefully apply the bead and smooth it out with my finger. If the sealant line gets messy, some thinner on a rag drawn along the edge of the sealant cleans it up nicely. Warning though: don't use thinner or any other solvent around your chromed plastic light fixtures, it will mess up the finish! I don't know why Oliver recommended silicone to you when most of what you'll be sealing was done with the 3M product. Good luck!
RE: the marker lights, I would like to hear how others have sealed those as I think I have a few leakers, and yes they were factory sealed with silicone! Can the chromed escutcheon ring be removed somehow for sealing?
Dave