We have travelled extensively with Kathleen's Manxter and had to leave it in many hotel/motel parking lots overnight. We have several layers of security.
1) master disconect (hidden) which kills all the electrical power.
2) locking Gene Berg shifter (locks in reverse).
3) wheel locks and run a cable through the wheels and to a post, tree or sign if available.
4) We cover the car, so without looking, they don't know what it is.
5) We try to park as close as possible to the office, so the overnight crew can see the car, and we ask them to keep an eye on it.
6) We tried an alarm that goes off when the car is moved and sent the signal to a remote, but it wouldn't work correctly, we are still working on this.
7) We have the later model bug steering colum, and we turn the wheels and lock it with the front end facing the curb, so you need a flatbed to take it away.

When we take really long trips, we have a locking storage area in the back seat area. See here
http://www.manxgallery.org/gallery/album254 for details.
If theives show up with a flat bed and drag it up, before we get the proper alarm, that would probably be the only way to get it. Tom