CCVYour concept is good, but none of the vans out there are designed for this. Transit / Promaster / Sprinter are all similar - Sprinter is slightly less actually.
The information is published in the body builder guides.
In addition to the roof strength, roll over is a factor.
Roof racks look cool, but do not add weight carrying capacity to these ratings.
The balancing act in general for these conversion vans is that roof space is so valuable. By the time you add enough solar panels to run the appliances and have some basics like a fan, antennae, etc, the roof is almost completely full - regardless of brand and model.
Kids don't mind sleeping in a tent on the ground. Just get one that has the 2 layer roof / fly for ventilation. In general they are safer closer to the ground than 10 ft up in the air.
HarryN has a good point here with a ground tent.
I have seen ground tents that attach to the side of the van & with the slider open & a van heater the tent might benefit from that van heat.