Here is an old post regarding the Trillium
Trillium Body Mounting Bolts
As I recall they were 5/16" which seemed small for the trailer, but they
had held for over 30 years so I guess they are properly sized. There
were 2 in the front and four toward the back all extending through the frame.
To determine the size of carriage bolt needed I took the old ones
to ACE Hardware and macthed them. Washers and lock nuts were used.
Hope this helps.
Regards, Larry H