I mounted my aux battery in a custom made box that gets bolted into a frame that supports my drawers and fridge slides etc, it is connected to the car via and anderson plug so it's removable.
I mounted mine on the passenger side to help counter-act the drivers side lean..
The last thing you want is your batteyr moving about so make sure it's secured properly AND fused at both ends of the cable, this is often overlooked...
I have a dual battery controller mounted in the recess behind the spare tyre and am currently experimenting with a DC-DC charger.
My wiring to the front is done with 35mm2 cable directly to the alternator, which IMHO is by far more suitable then connecting to the starting battery....