Westend - good and informative post - thanks. I also have a new trailer on which it was pointed out to me that there are some areas on the roof where the factory-applied sealant was "not sufficient" and that they could scrape all that off and replace it properly (at a cost of maybe $600!). After inspecting the roof with the adviser I said thanks for bringing this to my attention but I will do this myself. ;) The areas were as the OP described - joints at the roof-front and roof-back end faces.
I have Dicor self-levelling lap sealant already and my intent was, and still remains, to use this for small touch ups on existing sealant. Now that I see this thread and the comments regarding Eternabond, and after checking out their website, it is clear that I need to get some Eternabond as well.