If the trailer is a dual tandam axle unit (8 tires) then the reduction in capacity applies in this case just like a dual wheel axle on a truck. The modern tandam axle trailer (4 tires) with a equilizer bar has no need of its tires load capacity to be derated.
Many boat trailers (and other single and tandam axle trailer types) come with P tires. Fed 571 regs says this about P tires capacity on a trailer:
"When a passenger car tire is installed on a multipurpose passenger vehicle, truck, bus, or trailer, the tire's load rating shall be reduced by dividing by 1.10 before calculating the sum ....(snip).
I can't see Goodyear adding another 12 percent reduction on top of the fed required 10 percent reduction on their tires. They couldn't compete with other tire makers for OEM tire replacemnet, if this was the case.
