How can I keep food hot for extended periods without an active heat source?

The long drive is easy. There are 12 volt cooler/warmers available that would cover that need. But you have to do your homework. I found several that say they heat to 140° F or above, but I did see a couple that don't go over 135° F. There are also reusable hot and cold packs that can be used in an insulated cooler or carrier that will keep food safe for 4 - 6 hours.

There are several other options that could be used for a long drive or camping, etc.

• Instant, disposable ice packs or heat packs; don't require a heat or cold source and can be replaced as needed.

• Hotpots - Transfer hot food to the hotpot and it keeps hot for several hours.

• Hot pot casseroles - Food can be cooked in and carried or stored, staying hot for several hours.

Other ideas for camping:

• A chafing dish type setup.

• A camping stove.

*Both have certain time limits and shouldn't be left unattended.

• If you use the 12 volt cooler/warmer you can plug it in as needed. However you need to be sure that you don't run your car battery down. You can avoid this by running the vehicle while heating/cooling. (Not the most efficient.) Also, in some cars (like mine) the plugs will not work unless the car is on.

It looks like a popular solution is a good ice chest with heated bricks. I found several variations of this technique on various sites. It's an improvement over simply insulating since you have a lot of additional thermal mass and it's starting out at a much higher temperature.

You wrap the bricks in foil and heat them in the oven (e.g. 30 minutes at 450F) so that they're holding plenty of thermal energy, then line the ice chest with towels to protect it and place the bricks in. You might want another layer of towels around the food too, and you can place additional insulation over the top to fill any remaining space.

Depending on the food, I expect this could actually result in cooking it a bit further, but it'll definitely stay hot a long time. I haven't actually tried this and people's assessments of how long it lasts vary wildly. But given that ice chests can stay cold for a day or two in good conditions, I suspect that if you get the details right, this might well do the same for staying hot.

And then there is this one, out of Dutch antiquity. It really, really works.

http://www.rootsimple.com/2011/12/hay-boxes-or-fireless-cookers