I researched new tires for a year. I read various RV forums and forums used by freight haulers using medium duty gooseneck trailers. I chose to go with Michelins.
Many of the posts, especially by the freight haulers, reported very good luck with the michelins and most reported they got more miles from the tires than they expected. Keep in mind these are drivers that put 70,000 - 100,000 miles plus on the tires each year.
Many because they were happy with the performance of the Michelins were on their 2nd or 3rd set. If they weren't happy and didn't see the overall savings they would have gone with other brand tires.
The 400 may seem like a lot now but if the michelins out live the Firestones the extra 400 will save money in the long run.