Think of your 12v power as being like water.
Your leisure battery is like a bucket storing this water.
The alternator, solar panels and hook-up charger are like various taps filling the bucket at different times.
Using any 12v power is running water out of the drain hole in the bucket (yes, it's a special analogy bucket with a drain).
If the bucket is big enough, and not leaking too much, adding a second bucket is extra weight, expense and complexity.
However, if the bucket gets alarmingly empty at times, having a second bucket might help.
You need to be aware that if you are taking more out of the bucket than you are putting in, you will run out of water eventually.
A second bucket means it will take longer to run out, but it will also take longer to fill up again.
So a second battery can help in some situations, but not in others.