As usual, someone will jump on these threads and start calling people who try to describe how you would go about this "by the book" as WEIGHT POLICE.
It hurts NOBODY to know how you're "supposed" to do it before you decide to throw the numbers to the wind and do whatever you want anyway. At least the OP can make an educated decision. If he just wanted to guess, he could go to the dealer and let them blow smoke up his butt... He certainly would not be here.
The reality is, going by factory GVWR, your truck probably only has about 1500lbs of cargo carrying capacity. Pickup trucks are heavy. Diesel engines are heavy. It's a very simple thing to find out for yourself: Fill the fuel tank and take the truck to a scale. GVWR - actual weight = cargo capacity.
Ok, many people know anecdotally that a 2500 series pickup truck is more capable than a mere 1500lbs, even without suspension upgrades or tires. This is where you, the OP, needs to decide if you believe the people who make these claims. You don't know any of us from Adam. You don't know if we could tell the difference between a poor-handling truck and a rutabaga. For all you know we could be a bunch of Internet trolls goading you into going out and wasting a whole bunch of money, just so we can laugh at you.
Now, if you go to your tire ratings, they should be around 3100lbs each, and if you look at the white sticker on the inside of your driver's door frame, you will see that your rear axle rating is around 6100-6200lbs. From your weight ticket, you'll notice your rear end weighs about 2800-3000lbs empty.
Since nearly all of the camper's weight goes on the rear axle, that gives you 3000-3400lbs of camper capacity with the factory suspension and factory tires.
Keep in mind, though, that has to include EVERYTHING, not just the empty camper.
Since your truck is a short bed, the Lance models you can choose from are the 650, 825, 855S, and 865.
- The 825 has a listed "wet" weight of 2155lbs which includes fresh water, propane, battery, and leaves you quite a bit of capacity for your stuff and even a trailer. With just the camper you shouldn't even need any suspension mods on your truck.
- The 855S wet weight is over 3300lbs, leaving you with negative capacity. Hope you like camping alone, naked, and hungry... It's a heavy camper due to the slide.
- The 865 wet weight is 2330lbs, which again leaves you some wiggle room for camping stuff. With some minor suspension modifications and upgraded tires you could even tow a trailer.
The 855S is even a possibility if you go extreme on your modifications. This includes 19.5" commercial tires which have capacities in excess of 4000lbs each. Airbags, F350 springs... The possibilities are endless when it comes to beefing up the suspension.
Putting 10-ply tires on half ton trucks since aught-four.