+1

In addition to tensile strength, Stainless Steel seems to be a little more subject to galling and seizing of the threads. I've had stainless steel nuts seize on the bolts.

SS bolts might be strong enough for the application but I agree with Toyanvil. This is not a good place to use them. Grade 8 steel bolts are stronger and won't gall. 

If you use steel bolts then there won't be a dissimilar metals galvanic corrosion problem between them and the axle and chassis steel brackets. If you use stainless you will be setting yourself up for galvanic corrosion of the brackets because they will be anodic to the stainless steel. 

I'd much rather just paint the steel bolts, inspect them occasional for corrosion, and replace them if necessary than risk corrosion of the brackets, which would be much harder to repair. 

Depends where you live. Here on the Outer Banks with a hot, humid, salt spray environment, nothing metal short of gold lasts 70 years. More like 10-15 for steel if you're diligent about flushing with fresh water, painting, and using a water displacement material to fill all exposed cracks and crevices. I spray the bottom of my vehicles with Fluid Film a couple of times a year and that seems to help quite a bit. 

+1. That was exactly my point. You're creating a dissimilar metals corrosion problem where there wasn't one, and that problem results in the trailer and axle bracket steel now being sacrificial to the SS bolt. Since those are big bolts connecting to relatively thin steel its not good.