Water – Re-seal trailer roof

I have a small metal trailer. It is 5 years old and has developed a leak. I'd appreciate advice about how to re-seal it.

The trailer has a PVC coated galvanised steel body, about 6ft long and 4ft wide and high. I believe the roof was manufactured as a separate piece. There is a 90° bend at the end of the body and roof panels, forming a flange (if that's the right word) about 4cm long on each. The two flanges are riveted together inside the trailer. The join is filled with a white sealant (I assume silicone) on the outside. I've tried to show all this in photos below.

When I look inside, there is often water sitting on the flange (as well as on the floor and ceiling). I think that the two flanges have been distorted by jolts from the road, the sealant no longer fills the gap properly (or perhaps it is ageing & now lacks the flexibility required), and the water has a route in.

What would be the best way to fix this? Some ideas I've already had suggested to me:

• Add externally-rated silicone sealant between the flanges, outside and inside. (What sort of sealant would be best? Should I remove the existing sealant first? How? Also, could I really seal from the inside? The gap is mostly quite small and I don't want to make it worse.)

• Cover the gap up externally with a strip of vinyl. (But apparently the vinyl is unlikely to last long due to the movement between the roof and body.)

• Bridge the gap externally with P40 Glass Fibre Bridging Compound. (I have no experience with this… would it bond to the PVC coating? Would it cope with jolts, or shatter? Would it look neat?)

• Add more rivets?

• Cover up the trailer with a tarpaulin when it is at home.