I've actually been looking at the timer issue, as I plan on adding a second battery as well. It should be doable with a simple resistor, capacitor and a transistor, I haven't tested it out yet for timing. My understanding is that based on the component values of the resistor and capacitor, there will be a small delay as the capacitor charges before enough voltage builds to trip the transistor. Down side is that I believe there will also be a small delay in turn off as the capacitor discharges, so the solenoid would be held open after the 12v source is turned off. I believe this could be corrected for though by isolating the timer with a DPDT relay, and shunting it to ground to drain in the "off" position.
The problem though, as has been said, is that the solenoids tend to fail in the on position, which ultimately makes the timing circuit useless any ways. If it's failed "on" and you've drained the starting battery, you've drained the second as well. What's really needed before a timer is a circuit that will alert your to solenoid failure.
I have the parts to test out the timer circuit, but I plan on getting a winch, so I'd need the 500 amp solenoid. That's actually something I haven't seen mentioned on here yet though either. If you're going to run a winch off of both batteries, you'll need the higher rated solenoid, and the cheap 80A ones wont hold up to the current in the lines when you use it. A VR10000 is specced to use 502A at 10,000lb pull. It pulls 160A at 2,000lbs.
I've been trying to think of a simple way to tell if the solenoid is gone, but haven't been able to think of a way yet, I'm not an electronic engineer and I tend to over look things with circuits. I do know I want a small piezzo buzzer for a warning.
