After exploring the potential of the
Wonderbag, I ended up purchasing this
Tayama TXM-E60CF Food Warmer in One Thermal Cooker
A thermal cooker looks something like your Instant Hot Pot, but works differently. There is an inner pot with lid, which is removed from the outer pot and heated on the stove. The inner pot will usually be filled with a stewy-soupy meal, or water with the foods to be cooked in containers (Mason jars and glass food storage containers with silicone snap-lids are a favorite), or oven bags with the inner lid closed upon their tops (so you don't have to trust that zip-lock to be water-tight). The inner pot is brought to a boil and then placed inside of the outer pot, where the insulation keeps it above 140 degrees (and thus no nasties can grow in it) for 6-8 hours. Vacuum-insulated ones can maintain that temperature for 10-12 hours, but cost more than foam-insulated ones.
So, you can make a meal before departing and have it ready when you arrive at your campsite, or make it in the morning at your campsite and have it for dinner, etc.
The TXM-E60CF adds a feature: a 12V 35W heating element. This is supposed reheat the food on demand. I don't believe there is a thermostatic control, it's always-on. I suspect it can also keep the food near boiling continuously. This is an acceptable power level to run in your car while driving, or on your Trailmanor's solar panel during the day. It might run down a battery overnight, though.
Testing showed poor coupling of the thermal element, in the bottom of the outer pot, and the bottom of the inner pot. There's air between them. The thermal element got to boiling and the inner pot was cool. I had to pour a little water into the outer pot to couple them, so that the inner pot heated properly. Both the outer and inner pots are stainless steel, so that should be OK. I currently have that plugged into a 12V supply at home, and I'm seeing how long it takes to heat water starting at room temperature. I assume hours.
Although keeping a pot boiling doesn't take much gas, the usual savings quoted for thermal cookers is 80%, which seems credible to me.
So, this builds upon the hot-dog in Thermos method; which I have used for years, after seeing my European friends bring lunch on a day trip that way.
There are various recipes available, including a two-step method of cooking ribs in oven bags which takes at least 10 hours and leaves meat falling off of the bone as collagen has been broken down by long cooking at low temperature.
Baking breads is also possible, as is making cake from a mix, usually using a sealed container with the water not quite up to the lid. But one must compensate for the fact that no water is lost during cooking in a sealed container - one suggestion is to reduce water by 1/4 cup and add a tablespoon of flour.
Similarly, soups don't reduce. Start thicker, or use quick-gel (modified cornstarch) at the end.