There is no problem drilling into those. Just make sure your not going into the frame of your van from the bottom. Also once you start cutting holes for things like plumbing etc. Those are unavoidable. However, they making cutting holes into the van a tad more difficult because of the uneven surface. I found rocking the hole saw side to side helped the cut go very easy.
The floor is spot-welded to the frame cross-members underneath. You can see the lines of spot-welds on the floor inside; use them as reference lines. Spend time under the van to orient yourself before you drill through the floor.