George,
Your description of the excessive front tire wear on your coach sounds like excessive toe-in on the front axle. If you increase your front tire size, be certain that you have the clearance necessary for the larger tires. Several coach owners have increased the tire size on their coach and ended up with damage from the larger tires because there was not enough clearance, but most of those were SMC era Patriots.
As for as your question on Goodyear vs. Michelin for replacement tires. The Michelin tires will be more expensive, but worth the extra money. You may also want to look at Bridgestone. They are only slightly more expensive than Goodyear on average, but give much better service than Goodyear tires normally do.
Tire prices vary with some size tires, because all manufactures do not make the same tires (load range, speed rating, and tread design) in some sizes. So you will have to be sure that you are comparing apples to apples, when you are tire shopping.
As for replacing the rear tires, I replace my tires every seven years, regardless of the condition. On commercial trucks (18 wheelers), they are required by law in most places to be replaced before the cord body is 6 years old regardless of the recapped or original tread condition. For this reason, be sure that you get fresh tires by the DOT date code that is molded into the tire sidewall.
Gerald