I stumbled upon this, while removing some trim to fix a leaky roof.
If you are not looking for it, most often then not, you will not spot it.
So it is really important that you do a thorough check of your entire trailer on a yearly / bi-yearly basis.
Check the roof and the sides of roof where membrane folds down the sides, check for any tiny tear or hole. This can be fixed with Lap Sealant or Pro Flex RV.
Check the siding for any cracks.
This happens often, and is an easy fix.
Drill a small 1/8th hole at the END of the crack. Make sure it is at the end, other wise it will continue to crack.
Then fill will Pro Flex RV. You can use white, but I went with clear since the crack was also the decal pin stripe.