Good ideas here. We ended up using a cable lock and wrapping it through and around the hitch like glsBigSky. I had also emailed Sean at ProPride for his recommendation, and here is what he said:
"You can use a standard coupler lock to lock the hitch ball into the coupler. You can also snake the tow chains through the over-center-latches and use a padlock to lock it to the trailer frame.
Just to put your mind at ease, I have never had one reported as stolen and I've been in the premium hitch business for 18 years now. I think most thieves look at it and don't really want to mess with it."
I like that last part... thieves don't want to mess with the odd looking hitch!
By the way, this was our first time towing a travel trailer. We drove from Portland to north of Seattle, which is not a particularly challenging drive (except that we hit traffic all the way up). The ProPride is so smooth. No trailer sway whatsoever. We did have a small issue with installation, but that was fixed by the dealership. There is a bit of a learning curve on our part for the hitching and unhitching, but we are getting the hang of it. So far we are very happy with the ProPride.