Only thing I can recommend would be soak the shit out of the interior with an Enzyme Cleaner. That should help break down some of the "stain". Cat piss and gas aren't just odors on fabrics, they are actually more of a stain. They never really fully come out.
The only thing that has a chance of actually eliminating it would be some kind of enzyme cleaner. Maybe Bob or Kevin or another one of the detailers can give their opinion on this. Enzyme treatment, shampoo, enzyme treatment again, shampoo then dry. Make sure to put some fans on it or stick it out in the warm sun. Mold smells just as bad at cat piss.
I've watched a few episodes of "My Cat from Hell" on animal planet and this guy swears by this stuff. He gives a bottle to everyone who has a cat that pisses all over everything in the house and then he comes back with a black light to make sure they got everything.
Jackson Galaxy Recommends Fizzion For Removing Cat Urine | The Happy Litterbox