pooperscooper

I would find a reg that you can keep as close to 15 volts stable as possible without triping the overload. the more volts you can send to the inveter, the less draw on your battery, and the more effiecient your inverter will be. you might be able to find a small capacatier, say .5 farad, to act as a buffer from any spikes (15v+). this will also help with startup watts out of your inveter...microwave ovens and power tools, YES!!

CRXsi

4ga is still big wire...I still doubt you have any noticeable resistance to cause any noticeable voltage drop. Have you checked the resistance of the 2 wires to see what the actual difference is? I doubt it is of any concern.

Maybe the problem lies with the invertor? Do all invertors use a 15v cutoff? The other solution would be to look into installing a clamping diode or zenner diode to limit the voltage to below 15V...I don't know if you can get one for your amperage but an electronics store should be able to help.