I hear both good and bad about the Goodyear Marathons, though no personal experience with them. I do know they are the same tire as the Dunlap (I believe it is Dunlap, though my memory is fuzzy). Made in the same plant by the same folks and branded by both companies. You might just drop a note to the folks at Trailer Hitches & Vehicle Accessories (800)298-8924
and see what tires they recommend based on customer returns, comments, etc. You can also see reviews there and a number of other websites. Goodyear also makes a Loadstar line of trailer tires. Be sure to check prices for tires only versus tires and wheels - you can sometimes actually save a few bucks by buying them already mounted on new rims.