Valpo Camper,
Just out of curiosity, does it HAVE to be a locking fuel door? That might open up some options for you.
There's probably nothing special about the lock used and a local locksmith shop could probably install one on a regular door, if you didn't feel up to the challenge.
Personally, I think the locking cap is more likely to encourage a bad guy to cut your fuel line and steal the gas that way than to move on to another vehicle. In fact, that's even faster than trying to run a siphon. If they want to steal your gas, they'll get it.
Good Luck,
~Rick
