How can I increase the recommended camper capacity from 691 lbs to 2000 lbs?

Trade it!!! It depends on where you live (State/Province/Country). If you are in an area that limits your weight rating to the rating of your tires then you can swap for a higher load range tire (weight ratings are on the side). But you will be exceeding what the truck has been tested for safety-wise: brakes, suspension, steering, etc. In the age of lawsuits I do not recommend. As for the other half of the areas they set the weight limits as per the manufacturers' ratings, the GVWR and GCWR. In these area the vehicle cannot exceed the vehicle ratings for payload, trailer towing or individual axle loading (sticker on the door). Some area allow certified companies to alter but this is expensive. Some areas do not. In the long run your better off to get a bigger truck.

Look at what the heavy duty truck has as far as upgrades to your truck. To change everything out, like yetilikesbeer said above, you'd be money ahead to trade up instead of building up and get the truck ticketed. The highway patrol people are no dummies, and they have picked up way too many bodies off the roadways to let you go. You will get a more solid build, have a truck that can withstand the additional load, and be safe doing it.