It's not about peak output. What's it's horsepower at 2200 RPM? That's the measure of how much air it can displace at speed.
I don't like approaching GCWR for a number of reasons. First, you're pushing a fairly expensive truck to its max. That'll get expensive quickly. Breakdowns will ruin a vacation in a hurry. Having that little extra power is nice when merging into traffic. It's easier to drive and less stressful. Increased safety and control when needed.
I wouldn't think twice about making a 100-mile trip on mostly flat road at or even 10% over GCWR. Repeated 400-mile trips on steep grades with a little occasional rain, nope.
If you're going to buy a truck for a specific repeated use, leave some headroom. You know you'll always end up with a few hundred pounds of junk onboard. What if the next camper you really want is 1000 lbs heavier? Now you're looking at shelling out for a new truck too.
Note in Ford's towing guides they always say trailer weight with zero payload in truck and only a 150lb driver.