Hi Mark - as Markharf suggests, there are a number of resources, but nothing like as detailed as the British OS series unfortunately...
However, if it's recreation [paper]* maps you're after, then the National Geographic series of Trails Illustrated covers most of the popular recreation areas (and typically clearly defines hiking only vs. wheeled/motorised use too), and if you look on their website there is a map (#813) which covers the San Luis Obispo area:
https://www.natgeomaps.com/trail-map...aps/california
...which might be worth a punt? *note also that if you have the Gaia app, you can buy the National Geographic Trails Illustrated map layer for the whole of the US should you wish.
For general route planning, I'd also recommend the Benchmark Series of state-by-state Atlases... they don't cover the whole country, but they do cover pretty much everything out west, which is what most people want anyway ;o)
They have a huge amount of detail - which is also very accurate to what is on the ground I've found - including lots of tiny thread 'trails' which may or may not be suitable for vehicles - although pretty much any unpaved road/trail you'd ever want to ride a bike on is clearly marked. Perfect for overland route-planning!
Hope that helps...
Jenny x
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