Nice work, fun project.
I just installed an aux battery within the past year. How are you wiring your second battery? I used a solenoid that marries the two when the power is on. I have my winch and compressor isolated on the aux so I can use them power on or off.
After I put the aux in, I bought two new Everstart Maxx 700s from wallmart. I'm pretty happy with the setup.
j