Never unhooked on that kind of slope. (I drive a class A now but have towed many many trailers... assorted types.
First it it starts to roll away from you don't try grabbing the safety chain you are no where near big enough to stop the trailer. Grab the smallest thing you can find... the break-a-way cable.. that WILL stop it. Soon as it comes loose (Well a foot later).
Dual axle trailer try X-Chocks.
Single axle. Bigger chocks is all I can suggest and make sure they are SNUG against the "Down" side of the tire.
Let the trailer roll back into the chocks WITH the tow still attached. That way you can feel the block. DO NOT apply power.. Just let it pull you back onto the chocks.
