The problem you have is that this vehicle in on (your) property/driveway. If you have it towed off, then (you) will have to pay for it. Now, if you can push it off the driveway on a (public) street, then the police could have it towed to a salvage yard, then the salvage/tow would bill the GF for the tow and the daily storage fees.
Or, if you know where she lives, then tow it to her (new) place, thus she is stuck with it. Since she (originally) wanted to tow it from your place, then came and only took out the personal items, "implies" she realized that the van is worth (less) than the cost to tow it, thus left it at your place.