Protecting fuel tank from theft

Put a sticker(s) on it "Sewage Disposal Only".

If they really want it, not hard to pry a locking gas cap off and then a power syphon will make quick work of pumping out 10-20 gallons.

So other than urban legend, is there any evidence of the drilling method? I know catalytic converters are done but there's a big market and it's well documented.

As mentioned, if you are worried about it, find a secure place to store it.

I protect my fuel tank and catalytic converter by parking inside my locked garage. Not an easy option for many people, but indoor storage facilities are available, I've heard about people climbing fences at outdoor storage lots to take catalytic converters. Haven't heard of anyone breaking into an indoor facility, that seems like a lot of risk just to steal fuel.

Another weak point for gas theft is the generator fuel line (which on my RV is easy to get to and secured only by a hose clamp, but it'd be easy to cut if I didn't care about putting it back together).

If I ever needed to drain gas from my RV, I'd just pull that hose off and then I can drain 3/4 of the tank.

Park in a safer place.