In my world of OFF-ROAD CAMPING I would not want to do anything to interfere with the normal operation of my truck start system. It is designed for what you normally will use in your OEM setup... This is my only way to get back home on haha...
Adding things to the Truck Start systems might be too much load for your truck alternator, might run down your truck start battery along with the extra battery you have installed, might blow some fuses for the added DC current required for two batteries when being charged...
These are real issues to consider...
If you are convinced that the truck alternator is cable of charging two batteries at the same time then you might want to look into the SMART SOLENOIDS that are on the market that monitors your truck system system and will only switch in your extra battery when the truck start system is in its charge mode. It would not switch the extra battery into your truck system without the motor running i.e. Blue Sea SI-ACR 7610 and SUREPOWER 1315-200 comes to mind...
In the old days we used DIODE BASED ISOLATOR and those will not work with our present day of smart mode charging. The voltage drop across those high current diodes will keep your charging system to work with our modern day deep cycle batteries charging requirements.
I would look into these items from other sources to insure you know what you may be getting into...
No way would I just parallel another 12V battery to my truck battery terminal and wish for the best...
However this is just me saying these things...
Roy ken
