...........but I just can't let folks walk down the wrong path.
You want the skinny from someone who works on hundreds of RV roofs every year and uses cases of sealants, not just the DIY guys who do their own trailer and become weekend experts? (sorry guys, but I use more sealants in a week than you will use in your lifetime
major motor home or trailer manufacturer use one of 2 products: Dicor self leveling sealant or SikaFlex 221 one part polyurethane sealant. I see these on virtually every roof I am on, from small travel trailers to Million Dollar motor homes.
YOU WILL NEVER FIND SILICONE ON THE ROOF OF ANY RV OEM'S PRODUCT
I don't care what VTS or any other supplier says, USE SILICONE SEALANTS AT YOUR PERIL!!!!
The reasons can be found in many threads on this Forum, so I won't get into that.
Dicor is a good product but tends to show shrinkage cracking in a few years, necessitating the re-application of the product. I do this on a regular basis due to the brutal winter sun exposure in SW Florida.
SikaFlex 221 is a great sealant and bonding agent that comes in colors and once applied, you will never have to touch it again. Only down side to this is it must be mechanically spread (think plastic putty knife or rubber gloved finger) to properly cover the seams you are sealing. Almost any roof penetration should use a layer of butyl tape under the flange unless the manufacturer provides you with a gasket, like Fantastic or all of the roof air conditioner OEMs. In these cases, NO SEALANT of any type is used on the bottom of the device, just around the screw holes and outer seam on things like power roof fans.
Sika Corp. also has a new self-leveling sealant called SikaFlex 715. This comes in white only and for maximum effectiveness, the roof should be prepped with their special cleaner/prep solution before application. It is an expensive combination, but it is the best stuff I have found yet for this application. It will probably replace Dicor for all of my initial sealing tasks soon. It stays more flexible than Dicor and will not crack or shrink.