Help with Project

Did a search and found you guys. Hoping you can save me some time in my experimenting.

I have a trailerable houseboat. It's 40 years old.
https://lh6.googleusercontent.com/-B.../s640/duck.jpg
The original refrigerator is still working but terribly inefficient on 12 volts - it runs the battery down in about 10 hrs, although it will make ice and run happily on 120.

I bought a small chest freezer. The 'warmest' I can set the thermostat to is about 0°F. So I have wired it to a remote bulb line thermostat. Now I can run the freezer at any temperature I want from it's max low temp of -40 all the way up. I also have a run timer to keep track of the time the compressor is running and can monitor 3 different levels of freezer temp.
https://lh4.googleusercontent.com/-D...2/IMAG1388.jpg

The original intent was to freeze containers of water before we set out, and use the freezer unplugged as a cooler - not having to buy ice for the 3 or 4 days while we were out on the water. Brick freezing 5 gallons of water while the boat is on the trailer at home, I find I can go 5 days maintaining about 40° in the freezer without power...but now I wonder if I can do better?.

For argument sake, lets say beer is the sole occupant of the freezer. I want to keep it at 30°. What solution can I put in my 5 gallon container that will maximize my time without power and not allow my brew to get below 30° after the power is off? I can bring the temp of ice down to about -30°. Then what happens after the freezer is unplugged and the beer added - will the -30° ice bring the temp of the beer down below 30°? Is there something besides water that would work better for me?

Then, to complicate matters even more, how about 'recharging' the system for extended voyages? I've got my brick at optimum temp for departure, now after 4 or 5 days I'm wanting to recharge the brick while I run the generator (or boat engine). What's the best way to accomplish this without overcooling the contents.

And finally, would it be practical to use the inverter to power the recharge and run the generator or boat motor less often. The inverter has a 130 amp battery charger built in. I've got it set up for testing but haven't collected any data yet.
https://lh4.googleusercontent.com/-H...2/IMAG1389.jpg

The freezer really doesn't draw much - 2 amps @ 120? We run the generator on the boat regularly.