It's not indicated and certainly arguable, but with some application of common sense you could infer that the frontal area limit would be that which is EXPOSED and excluded from the front profile of the truck itself.
I've had a small R-Pod which loaded was about 3000#. Was still somewhat of a PITA to tow when fighting a headwind - transmission bouncing up/down trying to maintain speed, transmission temps climb, higher RPM overall etc.
I've since sold it and now tow my brother's pop-up. Weight is about the same as my R-Pod, but I can barely tell it is back there regardless of wind direction. It has a frontal area of roughly 32 square feet perhaps (just a rough guess), but it pretty well all sits behind the truck profile and pretty well is irrelevant.