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.
