Most states designate a salvage title with a different color and or a code or the word "salvage" or "rebuilt" to indicate a salvage titled vehical.
The question of "why is a salvage titled trailer harder to sell and worth less than another trailer" is an interesting one, considering most vintage trailers must be completely rebuilt before they are useable, are essentially (though not officially/legally), "salvage," yet they have clean titles. You never know what sloppy repairs/rebuild methods are hidden underneath that fresh vinyl floooring until you rip it up.
But, yes, there is a stigma associated with a salvage title, probably because of the used car market. A salvage title these days might result from vandalism that only resulted in some scratched body panels, busted glass, and smashed taillights, but who wants to take the risk that the airbags deployed and weren't actually replaced during the repair process?
A trailer could end up with a salvage title because the roof is dimpled from a bad hail storm, or because is has dents in several exterior segments that would cost thousands of dollars to replace. Then again, maybe it sat in 8 feet of water during Hurricane Harvey, and there won't be any obvious damage.
Buying any used trailer is a "buyer beware" situation, and you are best advised to educate yourself as to what "good" looks like so that you don't buy someones "polished turd." If your plan is to buy the trailer and use it, and there is no objective of reselling it in the foreseeable future, then a salvage title should help keep the asking price low, and actually work to your benefit.