Thanks!
I'm not sure offhand of a KML where someone has aggregated all public
lands, but if you are up to the task you may have more luck locating
shapefiles or other GIS datasets of these layers, and then converting
them to KML yourself.
In the meantime, if you are just wanting to explore the boundaries of
various public lands yourself in Google Earth, note that it has a
Forest Service and National Parks layers available. Simply expand
"Places of Interest" and then "Parks and Recreation Areas" under the
main Layers section.
Cheers,
-Josh