Don, it's not a simple calculation.
The basics are, it's X inches of snowfall, at Y pounds per cubic foot (varied by temperature, warmer snow equals more weight per cubic foot), divided by the slope of the surface, the steeper the slope, the more snow slides off.
The short answer is though, IMHO, a 17' trailer with a rounded smooth slippery roof doesn't have much to worry about, to err on the side of caution, build a simple pitched roof over it clad in the cheapest steel roofing you can find.