Appointments in Barstow for anything are mandatory. When I post previewed there I was only allowed at my truck, but they left me alone for 2 1/2 hours. Go with lot numbers you want to see. Now that they have two lots they are more controlling than I remember them ever being.
The best way to find them is to go to the main lot on the left and look for a forklift moving around. Otherwise get detailed directions to their office so you can sign in. It's in the warehouses on the left before the bridge. No matter if you find it on your own or find them in the lot, you will end up back there to sign in.
No idea how hard it is to clear security at the front gate. I get waved in without even an ID check at Marine bases. So make sure you sort that out before you get up there and end up late to your appointment.
Once in the gate you are on the correct road, no directions needed. Keep driving along that road until you see warehouses on your left and then lots of MVs on the right. Over a small bridge and the main lot with more MVs in on the left. DO NOT go to DRMO on your right. They are not user friendly. Do not try to find anyone at the lot on your right, that's shared with DRMO. If your lot is up there you will be escorted.
Unless you have the proper privileges do not stop at the gas station, Exchange, Commissary, etc.
Good hunting!