Well, you say you have put 30amps worth of extra accessories on a system that only has about 10-12 extra amps to spare. If this is true, then the extra battery is just more storage ability. Its like ..a 5 gallon bucket..with a garden hose and auto-shut-off filling it at the top and a 3" hole with a valve at the bottom. As long as that 3" valve is letting out more water then the hose is filling it, the level will drop. Changing it from a 5-gallon to a 10-gallon bucket only gives extra time until it's drained and doubles how long it takes to fill..or recharge as the garden hose..and the 3" hole are still the same. And as long as you are using at or near that 10-12 extra amps, the charging system will be at full output, making the rectifier/regulator work its ass off and get very hot.
I only see two options.
1: Increase the charging capacity to be at least 10% higher then the total usage. But no one makes a stator or kit that can go that high and no one makes anything for these anyway that I know of. However, several companies such as RMStator will take yours and rewind it and increase it by about 20%. That however isn't enough to overcome your 20 amps overage.
2: Install a voltage gauge and monitor it. When it gets at or below 12.5 volts, shut things off until the voltage starts to increase again and adjust your usage accordingly. If you start going through rectifiers and stators, you will have to consider reducing the load anyway. It's going to be on max output until it reached about 14.2 volts. That's when the bucket is at or near full.
I guess you could always put a small portable gas generator in back and power an extra battery charger...but...I really think you are not going to need or use 30 amps. That's a lot of power. All the fuses on the panel may add up to that but what is the actual usage total. You have to add up each item. You may only be 10-15 amps total in reality. And in that case, you may be just fine.