[H] dual batteries

[Winterlight]

I am going to install two batteries in my Silverado. This is not for 
a big audio system or anything like that. I want it because I power 
my laptop and run an inventor for small draw devices from time to 
time, and / or I use small devices when camping. I also want the 
batteries to be able to work together to start the truck if I am in 
cold weather. I want the axillary battery to be able to feed the 
primary battery as necessary.

 My truck has a heavy duty 145amp alternator. I got the correct 
mount kit from GM to be able to mount the second battery under the 
hood.. there is a special mounting place for it... so I don't need to 
worry about ventilation and a very short distance between the 
batteries and the alternator. I have heavy #2 wire. Then I bought 
this isolator http://www.winterlight.org/PAC200_manual.pdf however 
looking at this wiring diagram it appears to be for audio amps that 
are separately wired to the second battery and can not be used for 
axillary power to the vehicle. Looks like this isolator is designed 
to charge both batteries but keep both batteries completely isolated 
from each other which is the opposite of what I am trying to achieve.

If I don't use a solenoid one battery could just drain out to 
another...right? I am confused as to what solenoid I need to set it 
up the way I described. So what kind of switching isolator am I 
looking for?  Thanks