If you choose to isolate your aux battery and leave it in place, it is important to maintain (charge) it periodically to prevent it running down, which it will do over time. Allowing it to run down severely will shorten its life or ruin it, and when in a run-down state, it can freeze in cold weather and burst the case, which will likely lead to corrosion issues. Most batteries accumulate a nasty crud on their top surfaces over time, and this can accellerate self discharge and corrosion of cables and hardware. It's a good idea to keep the aux battery clean, whether you are using it regularly or not.