First off, ours is a 1980 Trillium
1300. That means that the frame is different from many of the earlier years; there are gussets welded to the "upcurve" right under the forward edge of the body to prevent the cracking often seen in earlier frames. Also, and I'm not sure if this is just on the later models or not, the forward two body-to-frame carriage bolts are not through the frame like the four in the back, but rather through a heavy steel angle bracket welded to the side of the frame when it was built.
So bearing in mind that other versions of the trailer may be different, here are the specs of the bolts I removed:
4 at rear: 5/16" mild steel carriage bolts 5" long, 18tpi, with square-punched backer plate on top and split washer and 1/2" nut on bottom.
2 at front: 3/8" mild steel carriage bolts 2 1/2" long, 16tpi, with square-punched backer plate on top and split washer and 9/16" nut on bottom.
Of the six, 3 were very badly corroded and reduced to a fraction of their original diameter, in all cases near the head where the damage was not visible. Likely this is due to exposure to wet plywood in the floor sandwich brought on by leaky window seals (fixed).
In other parts of this thread one user noted that his backer plates were round-drilled. In my case all six were machine punched with a square hole that matches the carriage bolt sub-head. This makes nut tightening down below easy: no spin up above. Maybe the square holes are on the newer Trills?
One user mentions installing the bolts with the nuts and washers on the top. I chose not to do this. My concern? I have seen split washers snap in the past. It's not common, but it happens. When it does, there is a danger of having the nut come loose. (At least until it is rust-seized
If this happens in the original configuration, the bolt will probably stay put. If things are upside down from the original, and the nut "walks" off from a failed washer and vibration, the carriage bolt or hex bolt will fall
out on the road. Remember that the load on these bolts is almost always "in shear", meaning they act to stop the body from sliding on the frame. I figure a bolt in the hole is worth a hundred on the road.
I ended up replacing mine in exactly the same configuration the manufacturer used, and I used the same type of mild steel carriage bolts. I did use some high-end caulk under the backer plate and around the bolt shafts where they went through the floor. On the rest of the bolt, I applied a generous coating of bearing grease.
My plan is to remove the body and clean and paint
the frame next year. When I do, I may reattach using stainless. If I don't get to it, well, I'm pretty sure these will last longer than I will.
As other users have said: if you haven't done this yet, there's still time before the snow flies. It took less than a day.