I'll bet it's from a smoker. Some hotels use an ozone generator in the room to try to clear it. Maybe you could find one of them and seal it in for a bit? Shampooing all the upholstery, especially the undersides, might also help. I doubt it's from the holding tanks. The black tank you would likely only smell when flushing the toilet. The gray tank you won't smell since the sinks have traps. What about also calling one of those disaster cleanup companies and see what they could do for smoke? I bet if you took it to their shop it wouldn't be too expensive.