I've flown with ADP before and most of their aircraft had working LORAN which is pretty accurate for a simple Lat/Lon so no GPS should be required. US maps have very clear lat/lon grids marked on them for quick reference. I flew around California/Nevada during Feb/Mar time and mountain flying never seemed a problem due to lack of any wind. Haze was always the biggest problem.
Bullhead city as already mentioned is a good base to avoid the smog of LA. Its got an very interesting steep taxi way! The only time performance seemed lacking was taking off from Grand Canyon towards the South (Density Altitude was 8000') and the climb out matched the rising ground below for the first 3 minutes, that was just the 2 of us. You can always plan your high altitude airfield visits early morning or at night when its cooler for extra safety. Just about every airfield there is available 24 hours with pilot activated runway lights.