How do I kill the last 20 percent of urine smell in a rental?

That last 20% is almost always hiding in the wall cavities or on the floor joists from underneath. If you only primed the top of the subfloor, the urine likely seeped trough the seams and is now sitting on the joists or the underside of the wood. If you have a basement or crawlspace access, you need to get under there and look for staining on the wood framing. If you see it, hit it with that same Zinsser oil-based primer. It's the only way to lock it in if you aren't replacing the wood. The drywall is another common culprit. Even if you primed the bottom 6 inches, urine vapor and wicking can go higher than you think. But more importantly, the smell is trapped behind the wall. Take off your outlet covers and sniff right at the electrical box. If it reeks there, you've got contaminated insulation or studs. You might not need to replace all the drywall, but removing the baseboards and spraying an odor sealer into the gap between the floor and the drywall can sometimes reach where the roller didn't.

You mentioned cleaning the vents as far as you coud reach, but that’s rarely enough in a soaked situation. If pets peed directly into floor registers, that liquid is sitting in the boots or the horizontal runs of the ductwork. You might need a proffesional duct cleaning company that uses a truck-mounted vacuum and can fog the system with an antimicrobial. A standard shop vac or a brush won't touch the crystallized salts deep in the lines. Regarding the ozone, one run ussually isn't enough for a house that was a total disaster. You need to run it for 24 to 48 hours with the HVAC fan set to 'On' so the gas cycles through the entire system. Just make sure you aren't in the house and you let it air out for several hours before staying there. If you're using a cheap unit, it might not be putting out enough mg/hr for an 1800 sq ft split level. You might need to rent a commercial-grade generator to realy finish the job.

Have you checked the actual doors and frames yet? I'm just starting out with my own fixer-upper, but I noticed the bottom of my bathroom door was like a sponge for moisture. If the pets were marking the walls, they definitely hit the door casings too. Since interior doors are usually hollow, I'm wondering if the urine could have wicked up into the cardboard honeycomb inside? It might be cheaper to just swap out the doors entirely if they're holding onto that last bit of funk. Does anyone know if you can actualy seal a hollow core door effectively, or is it a lost cause?

You relly need to look at the door jambs and the rough openings. That end grain at the bottom of a door frame pulls liquid up like a sponge. Even if you primed the outside, the urine is likely sitting in the shim space between the jamb and the wall studs.

Check if the bottom of the jambs are swollen. If they are, rip them out. You can't seal that away beacuse the odor leaks out from the backside where the wood touches the floor. I once spent weeks chasing a smell that turned out to be just two urine-soaked shims behind a bedroom door.

Check the hinges and the hardware too. If the pets were marking the frames, that stuff defintely seeped behind the metal plates. I once dealt with a rental where the hinge screws were actually corroded green from the ammonia. If you don't pull the hardware off and seal the wood underneath, the smell just keeps leaching out. Also, take a look at the door stops—those thin strips of molding the door closes against. They’re usally cheap pine and soak up liquid like a wick. It’s way faster to just rip them off and nail up new ones than to try and save them with primer.

Check the strike plate mortises. If there were large dogs, they often mark high enought for urine to seep into the latch hole. I’ve unscrewed those plates and found the wood inside the cavity totally blackened. It’s a raw, unsealed pocket that traps odors.

Also, pull up every transition strip and threshold. Urine always finds its way under those tracks and sits in the screw holes. If you only primed up to the edge of the transition, you’ve left a strip of dried salts right in the doorway. You have to get under those strips and seal that specific subfloor gap to kill the scent.