I hadn't heard from the manager, so I took the initiative to contact the gas station's boss myself. Thankfully he was in this afternoon. I introduced myself as the person his employee was helping out when the tank of gas got stolen. He was grateful for my honesty, as well as my concern that everything was okay. They got the idiot's licence plate and forwarded it to the RCMP; that's all he told me regarding that situation. I let it slip that the cashier had been spending the entire time on the phone (I was still quite agitated about the whole thing) and the boss sighed. He said he suspected as much because he had been trying to call the place on and off for a good 45 minutes that afternoon and the line was busy.
Turns out that the employee who was "helping" me was quite new and probably didn't know/care/realize that the cashier's till was being monitored by security cameras. Because the cameras showed this girl's actions -- a very, very lengthy phone conversation on company time with customers in the store to boot -- the boss knew that she wasn't paying attention to her job. That probably explains why I was never contacted about this incident.
Karma's a bitch, ain't it?
I don't know what became of the employee as I didn't ask (and the guy didn't volunteer that information), but I'm glad this whole mess is cleared up now.