The Rivian one was designed for Amazon specs. The Ford one is half the cost of a Rivian R1T. This is simply price economics. Amazon wanted to maximize space for packages, not batteries. They have charges at their warehouses and these are charged likely at night for off peak rates.

If we are talking about just an ambulance, the range could get higher, but there are diminishing returns eventually for a giant square so it probably needs an aerodynamic redesign.