In the boot, at the rear, maybe below the carpet will be the vents that allow the air to flow through the car. These should have simple flaps. Check they are undamaged.
When reversing, it is common to get fumes, as we reverse over our own exhaust fumes. I assume you have the problem when going forward too. If however you reverse off the drive first thing, you will get a lot inside. That may take a while for the smell to go. Could this be it?
After considering the above, as said above, check engine breather pipes and exhaust for leaks.
Problem still there? Does the tailgate seal properly? Put talc on the seal and close the door gently against it. Reopen and check it has connected all around.
You do have the heater fan on at least 1? Keeping it off will give you very little fresh air, retaining any fumes inside for longer.