I'd take a couple of flush mount tie down hooks and mount them on the wall. Take one of the exterior mounting screws out of the external skin, drill through (you'll be going through one of the chassis studs) the whole trailer. Put in a stainless screw and secure it with a lock nut. On the second hole on the tie down, use a self tapper. That should work. I've got plumming fittings on my trailer wall that hole spare wheels, when I have them.