I have no specifics on your breeze but as a long-time boater, the path is often mysterious, hence starting at the final point of leak. I would first check the obvious such as the gasket around the basement locker to make sure it is seating and water is not entering from the sides thru the door. Chalk line powder may help id spots that are not seated if the rubber is in good shape.
Has your friend gone onto the roof to check for obvious voids/damage in the sealant?
Most of the compartments on my RV are sealed except at the edges and where cables and plumbing enter. That would be another place to check for the LAST entry point which may give insight into where above to start looking. However, as it appears to be coming from the roof by your description and IF no leaks are seen inside, I would start looking at the edges where the roof cap meets the sides and any looseness or sealant issues in the gutters to coach, and bum gaskets in slides (were the slides open or closed?) and slide topper attachments.
Lastly, you can always try the hose on the roof (small area by area) to try and isolate the leak point
2020 Tiffin Allegro Open Road 32SA