Solar panel as a hatch and Is sleeping under a solar panel safe?
wonder if there are any chemical reactions or gases that will enter the van as a result of the back side of the solar panel working


No. Solar panels are inert. 


  I also know solar panels need ventilation underneath


Yes. A solar panel has a low albedo, probably much lower than the white roof of your van. So the panel radiates a lot of heat. 

Roof hatches on campervans are usually transparent. This has the advantage of bringing natural light into the living space, which makes the cabin feel pleasant and roomy. 
An opaque hatch only admits light when it's open, and only through the opening, i.e. light falls in from the side, not the top.

If you go camping in colder weather, you'll want insulation under the solar panel, to prevent the panel forming a cold bridge. Campervan roof hatches are double-pane to provide insulation. 

One more consideration: if you use a solar panel as a hatch, make sure the panel can withstand the aerodynamic loads. 


during driving, the roof shape, ventilation etc. may create a pressure difference between inside and outside
when stationary, opening the door while the hatch is closed may create a pressure difference between inside and outside, esp when the door is facing the wind direction and it's storming.