Trailer load capacity is trailer GVWR - trailer weight. A 20' 7k car trailer is around 2300 lbs. That leaves 4700. A 3515 weighs about 3000 lbs. Can't find the loader weight spec but I think it's about 1000 lbs. for my loader, the BL150 might be less. Box blades vary, my 66" BB is 550 lbs.
That puts you near the limit for that trailer.