There may be some misunderstanding. While the best goal would be to get access to 12v power without turning on the main system and drawing 300w, that may not be possible, or may cause too much load on the 12v battery, at least in the opinion of some. One would have to test it. I don't think this would count as "using the car as a stationary power source." This is powering a 12v accessory inside the car.In any case as I said, whatever api calls you use would just keep the car awake, and that would consume the same thing as sentry afaik. Why not just use sentry?
What I propose is not at all the same as turning on Sentry mode full time, which of course I would do if that were what I was proposing.
Failing a way to get 12v without going out of sleep, the next best thing is to make the car be awake about 1/3rd of the time during the night, and sleep the rest of the night. One could do that by tracking how much the car is sleeping (as Teslafi does) and waking it up every so often to keep it awake 1/3rd of the time. You can't force it to sleep but you can watch it and you can force it awake. The compressor will run and cool the fridge when the car wakes up. As long as it wakes up every hour for 20 minutes that's probably enough. That would mean 1/3rd the drain, so instead of losing 15 miles overnight losing 5, which is much more reasonable though not ideal.
I have suggested this to insiders at Tesla. They can do even better because they can watch the current on the accessory port. They could wake up the car, let the compressor run until it turns off, and then sleep immediately after that, then wake up again in 20 minutes and let it run as long as it needs to.
Better of course would be if they just had a way to keep the DC->DC converter on without turning on the computer. That would waste a little power but nothing like having the main system on.
Tesla debated having a 120vac plug powered off the main battery but decided not to do it. Now that other companies are putting that in, they will probably do it at least in some vehicles, certainly cybertruck. Perhaps an aftermarket add-on could appear for the Model 3.
Another option would be a $250 Jackery (costs as much as the fridge.) Or a $30 lead-acid could also do the job if you had a circuit that could charge it from the accessory port (never drawing more than 12a, and keeping the voltage on the battery at float level when it was full.) One could build such a charger fairly cheap, but I don't know if anybody does.