...please tell me why I need to vent the top? I thought propane gas sunk and that's why the bottom area needed to be vented Is it for the battery?
The gas emitted from batteries is hydrogen - it will want to rise out the top. The top-and-bottom vent recommendation is still probably more about the propane.
In both cases (battery and propane) it makes sense to me to vent both bottom and top, because whichever end the nasty stuff is leaving from, you need air to flow in to replace it.
Imagine opening a bottle of water and turning it completely upside down - the water certainly wants to drain out the bottom, but does not do so smoothly. With a vent hole punched in the top, replacement air comes in and the water empties faster. I realize that propane vapour is not like water, but it is still the denser fluid, and I think the top vent still applies.