Normally, I would agree with Lippert not having responsibility for a part failure on a 15-year old vehicle. And i would agree that the "recall" situation has gotten entirely out of hand.
However, we should remember that RVs are generally used far, far less than passenger cars. A typical 15-year-old car might have 150,000 miles on it and be on its second, third or even fourth set of brakes, while a typical RV may only have 5,000 miles on it, or even less.
Another factor is the issue here - cracked brake rotors. That this is a 'known problem' suggests that the rotors were under spec'd for the amount of heat they needed to dissipate. From what I've heard about Lippert chassis, this doesn't surprise me. Actually, what did surprise me was Lippert's offer to pay for the first axle. . . unless they did so to appease the OP vs. encountering bigger problems with a recall due to the "known problem" that somehow escaped NHTSA.
2008 Carriage Cameo 32SB2
2019 Ram 3500 Laramie Cummins 6.7 HO, Aisin
Rough Collie & Bichon Frise