You will have to locate the structure under the trailer's skin. By drilling directly into and through the studs, you will insure a strong mounting. Do not just drill through the wall at an unsupported area. One strong pull by a horse can rip the ring trough the thin material. I would fasten the "U" bolt using the largest stainless fender washers available, along with self locking nuts on the inside of the trailer.

Gard

I have a friend who spent 6 figures on a custom Dream Coach trailer with Dual slides. They installed a HiTie on the side of the trailer. 1st time they attached a horse, it pulled the HiTie right off the trailer leaving a gapping hole.  What a shame.  I have had HiTies on many trailers and had one horse get tangled and literial snaped the 1" thick fiberglass rod that makes up the HiTie, But the bracket on the trailer held. So if you install correctly as described by Gard. The trailer can hold a LOT of pull. If installed on just the aluminum siding, It can pull through the skin and leave a mess.

Locate the internal metal studs that support the wall and roof of the trailer and make sure your bolts go through those metal supports. I'd also add a plate on the inside to help spread the pressure over larger area than just a bolt head. Something like a 2" x 6" plate with the two bolt holes drilled in it. Or at least use large washers.

Trailers can support this type of pressure. Look at the ladders, hayracks, normal tie rings, door hinges etc that are currently installed on  almost every trailer.