Each build is gonna be different depending what you have in
the van and what you have installed on the van. I can feel the extra equipment weight when I have somebody join me on a trip. As I posted before, my van was heavy and even though I had sent the axle weights to Deaver, they still ended up having to send me an extra set of leafs due to the sag that occurred after the springs settled. They didn't charge me for the extra springs or the shipping. I think Deaver is starting to dial in their installs a bit better these days.
Weigh the rig with it full of fuel and estimate your loads or just fill it up with all your stuff you think will be on board.
Here is a good post on weighing your van.
Good luck on your purchase.