My first comment would be that it's probably not a good idea to mix tire sizes, even if they are not on the same end of the coach.
The other thing is that a six years is probably about as far as you want to go on a set of tires, so I wonder if the fronts were changed and the rears are old.
Here's a link that has some interesting thoughts on the matter.
http://www.motorhomemagazine.com/cforum ... 773678.cfm