As posted on the Yahoo forum...
Your 2551 is a light weight E450. The load on your E-rated tires doesn't compare to huge class Cs so you are not near the limit of the load the tires are designed to handle. Given your tires have no visual concerns, I would keep using them until specific issues come up which could be a number of years away. When you see significant (and I do mean significant) sidewall cracking, cracking in between the threads, uneven or severe tire wear, a bulge in a side wall, things like that, then I would think about new tires right away.
If you won't have peace of mind about it, then spend your money and help the tire industry.
But as for me with our 2007 2350 E350, it has the original like-new-looking E rated tires, now with 17,000 miles. I have no plans to change them until I see a better reason other than going by the calander. It does help that our rig is kept indoors when not in use.
Our first rig owned for 24 years, I sold it with tires that were 12 years old. It too had tires rated for a much heavier motor home. At the time of sale, the tires did start to develop minor side wall and in-between-the-thread cracks, but too small to be of concern.
If our rig was at the weight limit of the tires, I might feel differently.
