Some questions

[JimT]

How big is your alternator? that is, how many amps can it output? Everything running one the truck draws electricity from the alternator. if it's too small it can't provide enough amps to properly charge the batteries. each battery adds to the overall load. in general 3-4 batteries should be ok with a 170 amp alternator, however once you start adding things like refrigerators and microwaves, lots of extra lights and other electronics hooked up to an inverter, you may find that the alternator needs to be upgraded to safely handle the higher loads. Also, the more the batteries become discharged the higher the demand from the alternator. if the alternator is running at or near it's max for long periods of time it will wear out faster.

One other thing to consider is space. Batteries heat up when in use. Make sure you have enough space to safely and securely install another battery. Don't try to squeeze an extra battery into a box that was only built to hold 3. Upgrade the box or add another secure location if needed.

Try some google searches for things like '12 volt battery bank' and 'balanced battery charging' for more info. Lots of RV and marine battery related info out there that would also apply to trucking.