One idea are the powerline roads.

15 from Cajon north has a lot of parallel routes in the dirt with roads fine for 4x4,and staying on road 2 wheel is fine.

395 and 4x4 you can skirt alongside a long ways as the side dirt is well packed,I havent done that but I was looking last time we went to Kramer Junction then west on our trips to the coast.

The high desert is loaded with easy Jeep roads from offroaders that have played for years within site of 15,a mile or so in,more so on east side that Im aware of,so if you got stuck on 15 North its possible to drive right off in spots to side roads or past blockages on freeway.Cajon to Barstow,....we have property north of Victorville and the dirt roads and powerline road IS the access.Watch that loose sand though,stick to the rockier brown red colored stuff its well packed.There is also side roads in Devore area north on both sides.

For camp grounds look at city,county and state parks,we have a bunch of em and bugging out to a place from wilderness to powered sites works real well,we spent 2 weeks bugged out at Mohave Narrows,great place it was,power water and toilets and no pet issues.
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I cant speak on tire mileage ( but I dont discount it,being 'sticky' tires),I know (well Heard anyhow) the gasoline and diesel doesnt have the power they had a few years back.Wonder if thats a lot of what we see on that? In any event Im willing to pay that premium.Ive noticed since diesel change Ive lost about 1.5 MPG? Is that it or other things Ive done,maybe both? Dont know,any experts on fuel,or well read novices to comment?