Road from Manali to Leh is only open from Jun to September so you should do a good timing . I don't know about the cam base but near Leh ( ~40 km ) is Khadrung La pass which is the highest motorable road in the world ( 5602m - they said ) . I've been there before and it's very high already ( enough to have altitude sickness ) You also need to go to Yamauru from leh , i think it is one of the best road to ride .
Crossing border between Cambodia- Vietnam , Vietnam - China is sometime impossible . And as what i know now Tibet is " Taboo " .