You have to give the computers time to shut down, as well as the battery saver relay so your door and hood lights are out. Otherwise you're going to get a high reading.

When I'm testing for draw, I hook a jumper wire between the negative cable of one battery and it's battery post. Then unhook the other negative cable. Wait about an hour for all the computers to get shut down and for the hood and dome lights to go out. Connect your DVOM and then remove the jumper.

If for any reason you completely disconnect power, when you reconnect it all the computers will come up live and the dome and hood lights will come back on and you'll have to start over.