Thanks for the feedback, folks!
I'm going to pull the two "new" Marathon's I put on last year - I never asked for out of production tires or "cheap" tires when I bought the Marathon's. I've specifically said I don't want to be road-side so give me the best tire I can put on. The dealer I called last week offered Marathons, if they're no longer making this tire and me explaining what happened, that's disappointing.
I'll order 4 new D load tires. It sounds like Loadstar's are getting the highest vote. I'd like to stay with radials, looks like they're available. We've had some bad luck over the years with tires. I don't drive fast when towing, there's nothing really to cause it other than crappy tires. With all the tire issues, I check the pressure before every long trip. I checked the pressure last week an hour before it blew, it was right on 50.
Thanks again!