I've re decked a few trailers over the years and always stacked them tight, started from both sides towards the middle, and ripped the last board to exactly the right width on the table saw to where I had to tap it in with a hammer while others say to purposely gap the boards by sticking either an 8 penny nail or a 1/4" bolt down between the boards to keep the gapping even throughout. I've had trailer floors not last very long, have used pine, oak, at different times, doesn't matter.
The last (current ) deck is some species that started with an "H" but not hickory. Hemlock maybe? Not sure. I went to a local guy with his own sawmill that I was introduced to and recommended by many others to get wood for "special" projects like this. He said "this would be the last time I'd ever have to put new wood on this trailer". I had enough for a few extra/spare planks and I've used em up a board here and there at a time.
I linseed oil coat the snot out of the new boards as I put them on, I'm about to have to put the 3rd deck on my trailer since built in 2010. My trailer decks warp, crack and fall apart after a few years. I have a pile of rough sawn oak in my garage for the next go round and I m gonna try and gap them this time and see if it lasts longer.
I can lay down a tarp I guess if I have to haul something that im not wanting to fall down thru the cracks which hasn't been often
I use true dimension rough sawn lumber not "nominal" lumber yard sized wood.
At work we have a 10-12 ton trailer that was new in 2018, came with an oak deck, solid rough sawn 2" oak deck. Over the past winter the guys re decked it with more solid oak as the original one was beyond shot. I know they linseed oiled it originally when new (they happened to need this trailer 2 days later and almost lost the bobcat off the side trying to load because it was still so slick, shortly later they screwed expanded metal to it for traction) and they linseed oiled the new boards before they screwed them down... Seems like ~6 years around here is about the max life out of a trailer deck no matter the material or the attempted protection to try and preserve it longer... I admit to not recoating mine every year but I get around to it, probably about midway thru the life I get out of my deck boards...