When I bought my (new to me) used 1999 Class A in 2011, the previous owner had several boxes of older Thetford Blue packets under the bathroom sink. The boxes clearly said "contains formaldehyde" on the label. From several years of college level chemistry/biology lab experience, I know what that smells like and I'm sure they did. The smell reminds me of the commercial porta-potty smell along with the blue dye which is intended for industrial strength odor applications.
I used the PO's supply up then discontinued using odor control chemicals because the RV park where I was staying asked me not to. They said it wreaked havoc with their septic system. Since then, I've used only water and sometimes some fairly clean used dishwater with liberal amounts of dish washing soap already in it. As long as I dump/flush the tanks every 2-3 weeks, no odor problems at all.
Chum lee