Many government run parks have 'handicapped' sites.  Often although the
normal campsites are full, the H-sites are not.  Asking politely at the
kiosk/0ffice if you could use one for en-route (in after 6, out before 8)
overnight accommodations can often secure you a place for the evening.

Many parks also allow 'en-route' camping per se.  Check with the park
itself.  I seem to recall that Montana del Oro has this feature (MDA is on
the California coast near SLO)  TLA's abound though so watch out for 'em.