The California Sierras, Death Valley, the Mojave desert, and of course Baja (obv not in the lower 48) have remote trails that can take days to get to- not so much because they’re actually extremely far away but because the trails are slow going and difficult. There are trails where average speeds in a vehicle are slower than walking, but you can’t really hike them either - at least without pack animals- because there are no water sources.
Look at a map and see what is furthest from a highway, but technically accessible by some sort of old road or trail. Look at a mineral rights map and find the most remote gold rush era mining claims.
You won’t see people often in these places because your standard 4x4 lacks the fuel range to get there- gasoline engines have very poor range at low speeds. You need something capable but also fuel efficient- usually something with a small diesel engine, which are not very available in the US market. More remote places also tend to have a lot of cumulative water damage to trails so are very technical- requiring a lot of patience (e.g. stacking rocks for hours) and a lot of driving skill.
Interestingly, I’ve spent time in all the places you mention. Keep in mind more people live in Southern California than all of Australia.
The places you mention are busy, and not really remote. I’d be willing to bet you can drive out of the most remote on your list to a gas station in well under 10 hours. Probably 5
Okay, sure the longest offroad stretches I've passed through before finding more fuel have been about ~160 miles. There are places in Baja, California, and Utah that have trails of about this length between closest fuel stations. If you're talking about just driving out after you've already driven in that cuts the effective length in half - so maybe ~8 hours at 10mph average.
A fun thing about Baja is that often these super remote fuel stations end up being closed or out of fuel, so you can't realistically do it without having the supplies and fuel for at least 2x what it looks like you should need.
That's basically where I'm getting my "two days" from- the time from one fuel station to the next with about ~8 hours of actual travel per day.
