Lake Cuyamaca, Cuyamaca Rancho State Park, and Julian, CA are very popular vacation destinations in early June. Reservations need to be made early in your planning or you may have difficulty finding lodging for race weekend.
Lake Cuyamaca has rental facilities for campsites and RV's. Reservations and deposits are required from Genevieve at Lake Cuyamaca: Lake Cuyamaca Website. Or you can call at 760-765-0700 for reservations. Be sure to let them know you are with the San Diego 100.
Cuyamaca Rancho State Park (Paso Picacho Campground) is only 3 miles from Lake Cuyamaca and has campsites and cabins to rent: Cuyamaca Rancho SP (Paso Picacho)
There are several lodging options avaialble in the nearby town of Julian which is only 9 miles from the start. The Julian Chamber of Commerce website has many possibilities: Julian Chamber of Commerce
Also check out: Mt Laguna Lodge for accommodations in the Mt Laguna Recreational Area section of the race (about 18 miles from Lake Cuyamaca but near many of the middle aid station locations).