I'm a bit confused.
You keep the weight lower than 10k by not loading with more than 10k - is that the answer you are looking for?
Seriously, here are the steps I recommend;
You first look at the door pillar sticker and see what the Gross Vehicle Weight Rating is - the reason for this is even if someone here says what it is, you always check. It should be something like 9500 lbs, I don't have my Ford books here so I can't say.
*this part is optional*
Then you go to the nearest truckstop, fill the thing up with fuel and then weigh it.
Then take your GVWR subtract your weight and that will give you how much the thing weighs stripped.
*optional part finished*
Then you load it up with your equipment, bed room set and what ever else you need to have while you are on the road.
Then you go (*back*) to the truck stop and get it weighed (*again). This is your empty weight. Don't forget to stay in or next to the van on the scale when you do this.
You now take your GVWR subtract the latest weight and it gives you your cargo capacity.
For example;
My van has a GVWR of 9660 lbs and it weighs 5900 lbs with nothing in it and that gives me 3760 lbs. I put the etrack in, the little cubby sleeper and all the other stuff which now gives me on the second weigh 6950 lbs which gives me 2710 lbs of cargo capacity.