If you're going with a dedicated second battery for high-draw accessories, I'd say to wire in a Deep Cycle (dedicated deep cycle - not a dual-purpose) as your second battery, and run all the high-draw accessories directly to it.
This will allow you to run your accessories with the engine OFF directly off of the Deep Cycle battery, leaving your starting battery alone - and you'll be using the second battery in a manner more consistent with its design.
Be sure to use an isolator (OK) or a constant-duty solenoid (best) to separate the two batteries when the key is OFF. Ideally (I need to draw up the schematic and get it scanned...) you will wire it up with two override switches - one to cut the deep cycle in with the key OFF if your starting battery wants a boost, and one to cut the deep cycle out with the key ON in case it is "drawn flat" so you don't kill your alternator and probably the battery as well (the alternator will pop diodes from heat if it runs at full output too long, and you can "shock" a deep cycle by charging it up from nothing too fast.) This is quite easy to do - I just don't have the means to scan and post a picture as yet.
Constant-duty solenoids can be readily had at heavy truck supply houses - they're often used on work truck hydraulic systems to switch power to the electric pump motor. Don't use a "starting solenoid" (like old Fords) for this - they're rated for a 5-10% duty cycle, and tend to pop rather quickly. Specify a 100% duty cycle on your solenoid. I plan to start carrying them myself, once I get far enough ahead to do so...