I have the same issue the OP describes. This odor goes away while in use, but after sitting awhile, it comes back. If towels are other cloth is left inside, it must be washed - including things like pot holders. It was stronger when new, but it still lingers.
You are smelling the off-gassing of the cheap products used in the interior of your TT. Tank odor is very different than this. Particle board is manufactured with glue and who knows what else. Petroleum products are used in plastic and furniture - foam and covering. I don't think bowls of ammonia will help - you will be replacing one bad smell with another bad smell. You can rent a carpet cleaner and do the upholstery - I used one on the jack-knife sofa because someone spilled a beer on it. It no longer has that smell.
I never thought of charcoal - may try that. I usually open mine up on the weekends with all doors/cabinets/drawers open and run an oscillating fan. Products like Febreeze can help, the scented candle is a good idea, as long as it's not overdone.
Running the AC or using a dehumidifier can help too. You can get a small dehumidifier that stops when the the water tray is filled. Look on amazon. If you get an ozone machine like someone suggested, please let people know if it works for this issue.
Good luck.