IT totally depends on the trailer and manufacturer. Most lighter trailers have 3/8" bolts, most I've seen aren't even galvanized these days. I've got two Load Rite trailers here that use all 3/8" x 4" plain steel bolts to hold the cross members and bunks in place. Most galvanized bolts are soft, and SS is expensive which are likely the two main reasons.
You can use Galvanized bolts if you have a light boat, but if your dealing with a heavier glass boat, your better off with something more substantial.
SS is best, but if you intend to ever get it back apart use Never Seize on the threads. SS has a nasty habit of galling and seizing.
For most of my trailers I just use grade 5 mild steel bolts, I paint the bolts before use, and again after assembly and just keep an eye on them for rust.
Most of my boats are aluminum so strength isn't a big deal.
If your bunks are made from Pressure treated wood, the right answer is SS hardware, if not the bolt must be insulated from the wood with a plastic sleeve or coating. SS is the easier solution.
I get away with plain steel hardware mostly because I launch in freshwater most of the time, and when I do launch in saltwater, i make sure to wash down the trailer well, and give it a shot of saltx to neutralize the salt.
It must work, my newest trailer is a 1984, and my oldest is a 1967, both are galvanized frames, one plated, one hot dipped, but both are rust free after all these years.
