Good Year Marathon 225/75R15 Tires

Does anyone have an answer to the question as to how good Goodyear Marathon 225/75R15 tires are to put on my 2006 Dutchman Colorado 26 FB Travel trailer. It's got Duro Radial DS 2100 load Range D ST 225/73R15 tires on it now. I bought the trailer in Sept. of 2010 so the tires have got to be in the 6 to 8 year level where people say tires should be replaced even with lots of tread left and tires have no crack's or blemish's on the side walls. I know I need to look at the DOT number also to know how old the tires are. Thank's for any help you all can be on this matter!