Factory service interval for the 5R55E is 90,000 miles. IMO, too far regardless of vehicle use. I did a complete ATF fluid exchange with filter at 60,000 miles. Caught a special years ago at my local dealership that included 20 quarts of Motorcraft Mercon V, filter, and gasket for $199. Don't believe I'll ever see a deal like that again.
6R60 seems much more robust compared to the 5R55E and I'm not sure what Ford recommends. This also the same company that says "lifetime" differential fluid that doesn't require changing. Yeah, right. I've read anywhere from 30,000 to a 120,000 miles for the 6R60 on various forums, but earlier is always better regardless of whether you're towing or other severe duty use.