A grown Rottweiler is usually 80 to 135 lbs and measures roughly 38 to 44 inches from nose to the base of the tail. She's a compact, muscular dog, which means a lot of mass over a relatively short frame, and a lot of load through the hips, elbows and knees.
Rottweilers are commonly affected by hip and elbow dysplasia, and the breed is also predisposed to cruciate ligament problems and osteoarthritis. Again, not a certainty for your dog, but common enough in the breed to be worth taking seriously before she's limping.
The practical consequence is that a Rottie will flatten a soft bed fast. Poly-fill packs down under her within months, and once it does she's effectively sleeping on the floor with a blanket on it.
Egg-crate orthopedic foam is built to compress and rebound rather than pack flat, which is exactly what a heavy, dense dog needs from a bed.