Second the idea to check out the FMCA Michelin program. As to your wife wanting to only buy 5, that is a VERY bad idea. The question I would ask her is - if you aren't going to replace a 1999 tire in 2013, when does she want to replace it? One more year, two more years? Also you would have to put the 1999 tire on the FRONT since you only want to put matched tires together on each side in the back.
Show her the picture of the motorhome that went through the houses in UTAH this past weekend due to a blowout on one of the front tires.