Ideally: install a piece of sheet metal under the antenna (a few square feet) and attach it to that. The ground plane is there to basically act as a sort of RF counterpoise for the main antenna element; the DC voltage it's at is comparatively of little importance from an antenna performance standpoint (but may be of great importance to the circuitry in the radio or any amplifier circuitry in the antenna).
If you aren't going to create a proper ground plane, connecting it to most any handy chassis ground potential place is probably the best plan, but it may be worth experimenting with it connected and unconnected to see if perchance one works better than the other.
