Sounds like you are on the right track!My situation is a bit different, because all of the units have had a topper installed on the rear. In most iterations, the battery when in the passenger side toolbox. On this truck, it's going in the bed itself, mounted to the vertical wall we install to mount equipment. This will let me have a larger battery to drive the equipment, so I can do things like keep the emergency equipment running on most scenes without idling the truck.
On a non-topper-equipped vehicle, it would be a challenge. I'd either consider something in the bed (with proper restraints and a waterproof box) or consider something attached to the frame rail. Passenger side in the vicinity of the doors would be your best bet.