What follows is back of the napkin calculations, so please treat it as such and correct me if I am wrong.

1. Inverters are not 100% efficient. Let's assume 90%

2. Let's also assume that the user does not want to draw battery to 0 to not become stranded or have to do the "Honda generator in the trunk" trick. Extra 10%?

3. 300W continuous sounds a bit low even with gas appliances. Things like the fridge and furnace blower have spiky loads that push the daily average. Let's add 100W to the average load? I might be being too generous here, but I used 300W, not the 200W lower bound.

4. Vehicle side might need some consumption. If powering off the battery, it would probably need to cool the battery or keep some smarts on to make sure it does not drain or overheat? Genuinely not sure how to estimate this, let's neglect it for now.

Math is (50kw - 10%(inverter loss) - 10%(reserve)) / 0.4 = 100 (hours), ~ 4 days.

The above calculations assume a sane configuration (proper bidirectional wire, not suicide cord into 12v outlet). Quick skim of search for cars with bidirectional charging support for home shows batteries between ~40kWh(Leaf) to 250 kWh (Hummer).

So looks like one should be looking for ~80kWh battery, which actually most of the cars in the list have.

Again, very back of the napkin, would probably wanna add 20% margin of error.