A second 12v battery & this?

We're chasing a ghost load in our somewhat rough 2008 Prius.
To defend the 12v battery, and to be sure we have power when we need it,
is it as simple as installing a second battery in the rear drivers-side corner storage
and wiring this device between it and the existing 12v battery?

 

We've only just started to chase the problem - testing doors (if they're all latching correctly), interior lights turning off properly, etc.
I'll get our mechanic to attach his tester and tell me what he sees there.
This project was delayed to replace the tie rods, and now there's some looseness in the steering we need to resolve.
Before that it was the emergency brake (someone borrowed it and ran it with the brake on) - fortunately it was out of adjustment, so it wasn't meaningfully engaged.
Even with zero ghost load we still plan a second 12v battery - so that question remains to be answered.
Thanks.