If you have your outlet on the bottom of your tank it then means that if you are really low in the tank, you can place your vehicle at a specific angle to ensure that the tank outlet is always covered with water.
If your outlet is from the top, unless you have a fixed rod or similar going down to the bottom, you will not have that benefit.
As a footnote:- (Only if you have pure water in your tank)
If the outlet is on the bottom of the tank and providing you have a side opening door, you can, at the door, make a join on the pipe from the tank leading to the pump. When you then open that joint you will then have a gravity fed supply to feed your backpack