Slab City is just northeast of Niland, CA on the east side of the Salton Sea. Take State Hwy 111 to Niland, then follow "Main Street" out of town and a couple of miles past the railroad tracks. I don't remember the name of the military base - it was set up for WWII and dismantled at the end of the war leaving nothing but the roads and slab building foundations. Since the mid-60s it's been a self-governing community with minimum interference from the county government.
I've enjoyed my visits there. During the winter you'll find all kinds of people. Everything from the "slobs" as Terry puts it (people with minimum income who are pretty much stuck there) to more affluent fulltimers taking a break from the road. During the summer months you'll find mostly permanent or semi-permanent residents. Several RV groups like Loners on Wheels have semi-permanent encampments set up.
Like any other community, Slab City has it's good points and bad. If you go in and throw your weight around you'll probably have a bad experience. Try and blend in with the existing community and you'll probably have a good time.
If you don't like what you see when you arrive, there are several commercial hot spring RV resorts and a state park a little further north on Highway 111.
Tioga George has a nice
article about Slab City on his website.