The A400M is designed to operate from unpaved surfaces.
For the APU (number 70 in the following cutaway), it's space and proximity. You've got the wing box above the fuselage, plenty of space to add things there. Also easier maintenance access. Proximity as in less wiring and tubing to run inside the airframe, saves on weight and complexity. And FOD. The inlet would be away from the debris and dust the wheels kick up.
The RAT is indeed on the bottom of the fuselage, in the LH sponson just forward of the LH main gear, number 41 in the cutaway (source)
![A400m cutaway](https://cdn.statically.io/img/i.sstatic.net/WhAn9.jpg)