Good Question, First impressions are that they would be safer in the trailer. But if they are touching any metal surface, The lightning could pass through them.  but since the trailer is not directly grounded and typically is lower to the ground than the surrounding trees, The chances of it getting hit are reduced vs standing out in an open pasture. And the lightnig would probably pass around the outside of the trailer vs diverting through the trailer. but aluminum is a good condustor and a horse with him butt pressed up against the aluminum wall, could get hit.

I pack into the mountain alot and we just round the horses up from the meadows and high line them.   Usually in a group of trees.  First of all to shelter them from the driving wind and rain, and second to avoid any lightning.  If lightning hits a tree and takes out a horse. I guess it was just their time to go.  There is only so much you can do.  At some point it's either live with the risk or pack up and go home. And when you are 15 miles into the wilderness, some times that not an option.

My impression is that humans are more concerned about thunderstorms than horses are.  Some pretty nasty storms passed through yesterday--wall clouds with rotation, that later spun out some tornadoes and baseball-sized hail.  The horses only stopped grazing when it got really bad and just stood there with their heads down.  Before it had even let up, they were back to grazing, not concerned one bit.

I never tie to a high line, but I think a horse would be safer inside a trailer.  The trailer will act as a "Faraday cage" (I think that's the correct term) where lightning will conduct around the outside of the trailer and not through the inside of it.  A horse inside is like a bird on a wire.