We looked at dozens of RVs, and there isn't a spit worth of difference in the full size 5th wheels with the 12" Lippert Frame and 6 point leveling. The are identical on the Landmark, Bighorn, Montana, Sanibel, Redwood, Avalanche,.....etc. If there were engineering specs used, they copied each other. Even the gussets over the spring hangers are the same.
DRV does us 15" I beam, but it is a heavy dude. Given the additional weight, it's probably no bigger advantage than my 12" when it comes to weight to frame height ratio (which is exactly how they are engineered).
If you are looking at any with Lippert frames, whether it is warranted for full time or not doesn't seem to have anything to do with the design of the frame.