Can I make a neverending Soup in an instantpot?

I've actually tried it. It didn't work very well for me, but it might work better for you.

The problem is, cookers like the Instant Pot are designed for quick pressure cooking first and foremost. While they have a "slow cook" or "keep warm" setting, the heating element is still driven at high power, just at a lower duty cycle. Over time, that encourages solids to settle to the bottom and dehydrate and burn. (In contrast, a slow cooker operates at a lower power and heats to a lower temperature.)

However, the motion of the truck might actually help you out here by agitating the soup. If the soup doesn't have anything starchy (like beans or potatoes) that might be sufficient to prevent burning.

I agree with Sneftel's answer that the quality is likely to degrade over time due to contents settling and breaking down into stuff that doesn't taste good.

But just to add a thought regarding safety: food that's kept above 140F should in theory be safe indefinitely (see my answer to related question here). However, I'd be concerned about the proposed idea for several reasons:

• Does the "keep warm" setting actually maintain temperature consistently? (Hopefully it does, but many devices fluctuate quite a bit on such settings.)
• Can you guarantee that the cooker will basically have uninterrupted power?
• Adding ingredients can bring down the temperature of stuff already in the pot temporarily. This thermal cycling may become a problem if the overall ingredients go down below 140F enough times and spend time there when stuff is added (which could happen if you add a lot of new cold stuff at once). Some toxins are not destroyed at temperatures in the 145-175F range (or even by boiling), and some bacteria found in foods can form spore forms that can survive high temperatures and reactivate when food is cooled temporarily. Every time the overall ingredients dip below ~130F, you could start to see growth of bad stuff.

In general, I don't see a problem keeping this going for a few days/few batches, as long as you can verify it's staying at temperatures above 140F, and that the remaining ingredients aren't thermal cycling into the "danger zone" for very long. But for quality reasons as well as cumulative safety issues, I'd personally just try to finish the pot and clean it out periodically.

Considering this in the context of the earlier "everlasting stew" discussion, it's worth noting that there are certain ingredients which must be cooked using sustained high temperature. One notable example is red kidney beans which contain a protein which must be denatured (by boiling for ten minutes or so) before they are safe to eat.

The combination of some ingredients which must be cooked aggressively (e.g. beans) and some which are better cooked lightly (many fresh vegetables) or even eaten raw (for the sake of their vitamins) suggest that you'd be better doing your cooking in advance, chilling the portions, and thawing/heating when needed.

If you found somebody with a good stock of liquid nitrogen you probably wouldn't even need a refrigerator- just an insulated container.