We use Thetford Campa-Chem Natural Formaldehyde Free. It is a purple liquid and seems to work well. It contains BromoNitroPropaneDiol, whatever that is. I got it at Wal-Mart, I think. I see a lot of ads for Thetford Aqua-Kem and I may have used it in the past. I have some SupremeGreen and don't care for it much--tried it and didn't like it. Our smellers are not all the same and you will probably have to find something you can live with by trial and error.
As for toilet seals, I've read the chief enemy is harsh bathroom cleaners. The better black water tank chemicals have ingredients that protect valve seals, according to the ads.