The person did not add support to the roof for that vent. The roof is not designed to support a Maxx Fan or Fantastic fan.
The original 9 inch vent had a formed fiberglass raised flange around it, which both kept water at bay, and added support. Whoever did this job did not know what they were doing, and cut off the original supporting flange, weakening the roof further. Of course, to fit a larger vent in place, the original support does have to be removed. This is a CLUE that you can't just slap a heavier vent in place without support.
Check out the various Trillium
threads on this site. You can quickly become an expert on what to check. Obviously the belly band will need to be removed and fiberglassed. Window seals, door hinges, body to frame bolts, are all areas where Trilliums need attention.
On the floor, other places to check are under the refrigerator
(open the outside vent cover). Any soft spots anywhere are jumbo bad news on a Trillium.
The black stuff you see around the vent tells you this is NOT a recent problem! Water has been pooling up in that area for an extended period. The best installations I have seen involved extensive fiberglass work on the outside of the trailer.
Seller should have left the original vent in place instead of botching the job. Of course its too late now.