Check carefully around the edges of the Dicor where it meets the roof. If it's cracked or visibly lifting that's a sign it should be replaced. A void will often appear to be black or very dirty. It's important that all services, including old Dicor are clean before applying more. I'd be cautious and only apply new sealant where it's needed.
Remember, much of The Dicor you see is superfluous. It's the sealant between the fixture (vent, etc.) and the Dicor on top of the mounting screws that's doing the work.
I come from a sailboating background and I've never understood why the RV industry glops on sealant by the bucket, while on a boat, you almost never see any sealant. At most, you'll see a small bead around the fixture like you might find around your bathtub. And, if anything, boats are subject to much worse conditions than the typical RV. It's the mounting compound underneath the fixture or in the screw holes. I guess it's just faster to apply in huge globs from a caulking gun.
Keep in mind that the Dicor "lap sealant" you'll use on your roof isn't the same as what you'll use for any vertical surfaces like your sidewalls. Winnebago publishes sealant "callout sheets" for their motorhomes that detail what sealants to use where, but, unfortunately, not for their towables so I don't know what to recommend. I think this is a great disservice, along with the lack of wiring and plumbing diagrams, etc.
Some of us use Eternabond tape to seal our roof fixtures:
https://www.amazon.com/eternabond-ta...ternabond+tape
Here are links to some videos and articles. Like many things, there are those who are dead set against it but it's served me well. On the other hand, my 35' motorhome roof is a bit bigger challenge than were my two smaller TTs on which I always used Dicor:
https://www.google.com/search?q=eter...hrome&ie=UTF-8
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