The are probably stress cracks caused by flexing of the trailer.
At the end of the crack, drill a small hole - 1/8" is plenty - that will stop the crack from growing. Then get some good white vinyl tape and cover it to keep water out.
I had stress cracks above the forward door of our '99 Sunline. Drilled 'em, taped 'em, and enjoyed another 9 years with the trailer.
'11 Cougar 326MKS loaded with mods
'12 Ford F250 SuperCab 6.7 PowerStroke Diesel
KD2IAT (146.520) GMRS:
WQPG808 (462.675 TPL 141.3)