Unity's LODGroup decides which mesh gets drawn. It still keeps every level in memory, including the ultra-detailed one you will never walk up to.
Vicinity handles the other half: what exists in memory at all. Objects load as you approach, and are released when you leave. The two systems work together, they never fight.
It is v0.1, fresh out of the oven and not battle-tested yet. Source is up, feedback welcome.
github.com/Nekuzaky/Vicinity