Picking a good location will help maximize storage space and the unit's airflow requirements.
Consider an area with good ventilation. The unit has a circulated air intake that pulls air from inside the van, a supply air intake and a supply air outlet that goes through the floor.
Consider where you plan to run your ducting. The shorter the ducting the more efficient the air conditioner will be.
Check beneath your vehicle is clear from obstructions as the intake and outtake pipes sit proud and need clearance
Keep in mind things like river crossings and ensure you have enough clearance.