Here's a simple trick I do when it comes to grounds on a trailer. I get some stainless steel 1/4" bolts and drill a hole in the steel where a ground is needed. push the bolt through the hole and weld the head. The threads will be on the other side of the steel so they aren't going to get damaged by welding. Now you have a stud to bolt your grounds to that will never loose contact with the steel frame or rust. The weld isn't stainless but as long as you paint the weld (like all the rest of the trailer) it'll last. Just don't paint the stainless threads.