What size truck will I need to tow a 28,000 pound trailer?

You are certainly in the Class 6+ category. I'm seeing more Freightliner M2 crewcabs, but a class 7-8 is not out of the question. You're really pushing the availability in the used market with the bed setup. Don't be bothered if the class 8 has many miles, that's what they're built for.