Has anyone tried this? I figured they would last a lot longer and hold up better. My main concern would be the lag bolts not catching as well as they would on lumber.

Maybe a counter sink with thru bolts?

Thinking of something like Choicedek?

I put side boards on my trailer used composite , works great no rot or split been there about 5 yrs.

I haven't used any composite for trailer boards, but I have built serverial decks with it. I would be iffy about using it for that purpose. I have found that the composite isn't nearly as strong as a 2x4 compared to the same free span. I think you could get a pontential failer in between the trailer mounts.
The lag bolts would hold just as well I think.

Thank you, yes I was thinking of something simalar to choice deck. Replacing bunk boards is a pain and I don't want to have to do it again for a long time.

I used the treated lumber on mine when I replaced them which should last alot longer-flat bolts with washers and nuts hold better than lag bolts,