In the long run, the only thing you can do is make your RV less attractive to thieves than someone else's. When I pull into a rest area, I get out to stretch my legs - by locking my rig and walking around it. I don't walk into the bathrooms there or anyplace else. I use the one in my rig.
If I need to shop, I do so in my Jeep toad after I'm parked in a FHU RV Park for the day. Generally, they aren't as attractive to thieves because there are people out and about and there's generally only one road in and out of the park.
Wally World OTOH, has people going in and out all day and night. Another car driving around the lot doesn't draw anyone's attention. Same deal with any large retail commercial building.