It all depends on where you get this 3000/3100 number from.
If it is off the yellow door sticker in the EXACT truck you want to buy, then it is legitimate.
Otherwise it is just advertising fluff. "Up to" and "when properly equipped" are certainly in the fine print.
Just be aware that things are never as simple as they seem. There are always "gotchas" lurking around the corner. All truck and RV dealers care about is getting you to sign over a bunch of your money. They don't care if you're happy or safe or compatible. Advertising numbers such as "1500lb dry pin weight" or "3100lb payload" are used to get you in the door.
That said you may or may not be perfectly happy with how things are even if you're a bit overloaded. The problem with that is it's a $100,000+ gamble whether you will or won't. When that much money is at stake doesn't it make sense to err on the side of caution?