If you have a deep cycle Gel Cell battery such as a yellow top Optima, you can run the thermo electric coolers for 7-8 hours with the truck turned off (assuming all other accessories are off!) and the truck will still start. Expect maybe 4 hours with the regular battery.

Frankly, I'm not impressed with the thermo electric coolers. They are fine if you're just cooling pop or beer, but the reality is they don't keep things cold enough, so in my experience perishables spoil very fast. For a weekend trip it's fine, but using one for an entire week of wheeling is an excersize in rotten food, blown fuses, and melted wires. I found myself often adding blue ice packs and such with my perishables to help keep them cold. It also takes about 6 hours for them to cool so you need to turn them on the night before you leave or make sure everything you put in there is cold to begin with.

Remember, at best these types of coolers cool to 40 degrees below ambient temperature and that's really pushing it (in reality it's more like 30 degrees). They have very poor insulation too, so once they turn off they warm up inside very quickly.

I have one of those 5 day coolers or whatever they are that takes ice and it works much better than the thermo electric.

However, I have now graduated to an ARB Freezer/Fridge. IMO, it is totally worth the expense. It will freeze food and it only takes 1/2 an hour to get to refridgerator temperatures. It also draws less power than the thermo electric coolers and is better insulated so it stays cold when turned off...especially if you get the insulation bag for it. Yes it's heavy and expensive, but so is rotten and unedible food!