Well first I have to ask,what is that bar on the roof about?
I have not seen that before but it looks like someone has already explored what you are asking about?
I would get a pole or 2x4 and just stick it inside to return the roof to normal and let it stay there while you are not using the trailer.
Maybe more than 1 depending on how bad and how many spots seem to sag on their own.
I think I would position it right under the bar outside and see if the roof looks better with it in.
On my Trill 1300 I installed shelves inside above the windows
and then jacked the roof up somewhat with little braces from the shelves. It seemed to redistribute some of the roof load to the walls and shelves and my vent puddle went away outside too.
Hope that makes some sense?