TomDiesel

The idea of using a separate battery to power lots of neat electrical stuff, but not drain the battery you need to operate your truck has a lot of merit, and has been done before. In fact, the separate battery attached to the truck battery by an isolator diode is the simplest solution. Having to recharge a deep discharge battery when it dies is a lot of work. After all, how do you know when it needs recharging? Then there is the connecting and disconnecting the charger (After opening the hood, of course). Using the isolator guarantees the separate battery will always be recharged once you start the engine. Let me explain. A vehicle battery is considered charged up when it is at 12.6 Volts DC with the engine off. Your alternator puts out a voltage greater than 14 Volts DC when the engine is running (How high depends on battery state and other elecrical demands). The isolator diode will charge the separate battery once the charging voltage is greater than 0.6 Volts DC greater than the separate battery voltage. If the separate battery had been discharged down to 12.4 Volts DC, that would happen at 13 Volts DC.

By the way the engine code letter is the eighth digit of the VIN code

Code P0455 is normally seen with a loose gas cap. The symptoms you describe do not appear to relate to the code. What indicators do you see on your dash?

You should be fine. The minimum payload listed for a 2011 Sierra 1500 is 1494 lbs. The maximum that I found is 1937 lbs (depends on how the vehicle was spec'd). If the trailer tongue weight is 800 lbs, you will have room for 694 lbs of stuff. If you have the beefier model, you can pile in 1137 lbs of stuff in the pickup. In either case, that's a lot of stuff.