Eggbert was my very 1st tiny trailer a 3 x5 motorcycle utility trailer. I coated him inside and out with "Polar Seal" He has resided in my back yard since 06 in all types of weather without any signs of degradation of the cheap 7/16 osb that I built him out of. I am not sure I would use it as the final roofing material but as a preservative it has performed beyond my wildest dreams.
Gypsy trailer roof is coated with an elastomeric seal. I did put down 3 coats of CPES first though. Nice thing about the white roof coating is that you can tint it any color you want.