Do NOT buy a locking gas cap!!! Let 'em try to siphon gas out of your tank... unless they brought a really loooooong hose it's not going to happen. Thieves want to do things fast for fear of being caught so if they try to siphon and can't they will probably leave. If they do succeed they will usually only take five gallons but if you have a lock there is a good chance they will simply puncture the tank... get what they can and leave the rest draining on the ground
(not good under a motorhome).
ps I much rather lose five gallons of gas than try to repair a hole in the tank!