Building a parking for the trailer and cover suggestions

I don't know if weeds are a problem where you are? Around here, weeds are proliferous and will even eventually grow in just straight 3/4" crushed gravel (no "fines"). If weeds are an issue, I would suggest using filter cloth (the heavy duty kind) on top of the soil before placing gravel unless you use something like crushed limestone.

Maybe crushed recycled asphalt would work? They call it "RAP". Packs down nicely and can be removed.