If you check out where the major frame members are for your trailer then add some extra piers underneath and place your safe above them, weight should not be a big issue excep for maybe getting it in and out. Since most trailers have particle board floors you might want to lay down some planking when you haul it in. As far as fire resistence 1/2 hr should be more than adequate. with trailers there is usually nothing left to burn at that point.
