Some trailers have a piece of flat bar, steel or aluminum bolted or welded to each crossmember. It is called an “under frame tiedown flat”..
Otherwise you are usually just going to hook to the deck frame outer rail, or lace rail, which is usually C-channel. On lower end gooseneck’s, some lace rail is just 6″-8″ flat bar..
PJ’s in particular used to, might still use tube for the lace rail. In which case you can either spec the under frame tiedown flat, or you got to hook to the rub rail. A few chain tail straps come in handy so you can grab pockets & spools as well..
