As mentioned, depends.
Does it need to be somewhat structural, or can the trailer sit on it now without it sinking, getting soft (even in the rain and spring thaw)?
If so, just 3-4" of crushed rock to pretty it up.
If not, need 6-12" of road base, compacted, then cap it with a little gravel.
Even softer? Layer of fabric/geogrid then base and gravel.
Best to put down a vapor barrier for 2 purposes. Keeps excessive moisture out of the subgrade and controls weeds.