We have but 45000 miles. I haven't even looked up the recommended interval, but I don't think it was 100000 in the earlier years. My guess is Ford upped it from experience and for publicity reasons. To me that makes it something I can do at my leisure so to speak. One philosophy is don't give the plugs 100000 to have time and miles to really plant roots.
Be assured, I'm approaching this with extreme caution. How extreme? We bought this coach in 2008 and I have YET to take the doghouse off. Figurin' it won't bother me if I don't bother it...