The payload capacity of your truck = GVWR - scale weight. It doesn't matter what any sticker, salesman, or brochure says. If you want to know the payload capacity you have to weight the truck.
Now, what sticker are you looking at? Is it the one in the glove box? This one assumes that you have a 150 pound person in every seat in the cab. If this is a 6 passenger crew cab truck then they have subtracted 750 or maybe 900 pounds form the payload for passengers. (I don't remember if they count the driver or not).
If you're look at a '90s Chevy the GVWR is probably around 10,000 and the truck probably weighs about 7,000 empty so you've only got around 3,000 payload. Subtract the passengers and you're down near the number you gave from the "sticker". Yes, it's really sad but that's all that truck is rated to haul.
If you're looking at a 2001 - 2010 Chevy the GVWR should be 11,400. The truck empty probably weights about 7,500. This should leave you around 3,900 pounds of payload. Even subtracting for people should leave over 3,000 pounds for payload.
Did they "use it up with a heavy truck"? Yes. Crew cab, diesel, 4wd, it all adds weight. If you look at a 2011 and newer Chevy the dually should have a GVWR of around 13,000 which helps your payload considerably.
Now you know why the majority of trucks hauling TCs are over their GVWR. Most of us go by axle and tire ratings. My own 2005 Chevy dually is rated for 11,400 but scales at about 13,200 with my family, 11' TC, and boat hitched up. I'm not over the ratings of my tires or axles. I've got many thousands of miles behind me with this set up.
Is it legal? Absolutely, my truck is registered and tagged for 14,000 pounds. I know a lot of people will try to say I'm going to kill someone, I'm going to get sued, etc. That's their opinion. Many many professional "hot shot" truck drivers run dually pick-ups all day everyday over their ratings hauling automobiles, pipe, etc. They are routinely stopped by DOT for inspections and cross the scales at weigh stations. It's perfectly legal.
Trucks can be modified to have a greater payload capacity, not a greater GVWR mind you, but a greater physical capacity. Only licensed up-fitters can change the GVWR. I once saw a truck that started life as a 1500. It had been modified with tandem rear axles, a 500+ cubic inch engine, and a fifth-wheel hitch. It was used to move semi trailers around. I bet the door sticker still said something like 6,400 GVWR. In reality that was capable of much more.