JFI....All trailers don't have two chains but one chain attached with a fastener in the middle of the chain. Some trailer required the single chain to be looped around the skid plate on the bottom of the tongue.
Most trailers today probably have two chains.
I use 3/8" gr 70 transport chain on my 10k car hauler (two chains attached to 1/2" loop welded to the bottom of the trailer tongue. And 3/8" grade 70 slip hooks with safety latches.
One area I see some rv folks get in trouble is using a good chain with the proper load capacity but then use some type of "hook or latch" that doesn't have enough capacity.
When the trailer leaves the ball its shock loads that can break a chain or a hook when it falls or a sudden left/right movement.