My understanding of frame stretching is that Ford published a standard that the motorhome builders are following. And that the standard if followed preserves capacity and warranty.
That said, I have two observations:
1. There's still a lot of bickering when it comes to chassis warranty issues like driveshaft alignment and balance.
2. There may still be a down-rating of hitch capacity. It only makes sense that a hitch 10-12-feet behind the rear axle should not be expected to carry the full rated tongue weight. Not uncommon for the max towing capacity to be for flat towing, which applies essentially zero weight on the hitch.
So what I'm saying is the "7500-pound rating" may be for towing a vehicle four-down or on a dolly, and does not allow for the tongue weight of a 7500-pound trailer.
I've also read that Weight-Distributing hitches are not recommended for stretched-frame motorhomes.