Good post, I was just wondering the same thing after this past weekend driving up to Oregon with three bikes. We didn't stay at a motel because it was an 8 hour trip, but when we stopped to eat at Denny's we always got a table by the window where I parked the truck.
For some reason a lot of people stopped to look at the bikes and I just had to wonder about what the heck I would do if I had to stay overnight somewhere.
I thought of a tarp as a good deterent. Also, running a chain through all the bikes and then locking it to the truck, all suggested above. I guess someone would have to go to a lot of trouble, but could still successfully rip you off!!
How much do those Wells Cargo trailers run?? That's crossed my mind as well.