I read that Ruslan Chagaev is Valuev's next mandatory challenger which would make their matchup the first all white heavyweight title fight (admittedly with non-unified belts) in how many years?
Actually it's not quite as long as you may think. Valuev fought Vidoz in October of 2004. Vitali Klitschko fought Corrie Sanders in April of 2004, and Wlad fought Sanders a year prior.
Valuev vs Vidoz was for a regional belt. Recent all white title fights in the heavy division have been,V. Klitschko vs Sanders(for the new lineal belt), W. Klitschko vs Sanders, Botha vs Shulz and when the WBO was was a fledgling joke organization Damiani vs Giovanni and Damiani vs Duplooy.
Marciano knocked out Britain's Don Cockell in 1955, in what was long considered the last all-white heavyweight title fight. Marciano stopped Roland LaStarza in the 11th round in 1953. Marciano-LaStarza II (Rocky had decisioned LaStarza previously) was the last heavyweight title bout between White Americans.