I haven't used or found a need for any additives on a regular basis in our MH... just plenty of water.
I have had issues, mostly strong foul smell from our boat holding tank and have learned a lot solving the problem. Inadequate venting to get air to the tank and liquid surface is key to eliminating odors. Aerobic bacterial breakdown is not bad smelling but starve the liquid of oxygen and anaerobic bacteria take over and they are the really smelly ones.
I have improved venting and even went so far as adding a fish tank bubbler to the tank and have had great results.
I also started using a product called NoFlex which is relatively expensive and hard to find. The active ingredient is sodium percarbonate and is inexpendive and readily available in bulk. It only takes about a tablespoon or so in the tank each day to keep it aerobic . The treatment not only aids breakdown of solids but also cleans up residual solids and tank residue. The key is a small daily dose and a 2X-3X dose if you are leaving it unused for a week or so.
I also use Calgon periodically to assist the cleaning of tank and level sensors.
If anyone is experiencing significant odor problems id recommend giving sodium percarbonate a try. If you buy a 15# bag / container you will have plenty to use and give some away to friends & acquaintances.
If you need or want more info on dosing & use PM me and I can elaborate and provide additional links & info.
Don & Marge
'13 Newmar Ventana 3433 - '14 CR-V TOAD
'03 Winnebago Adventurer 31Y - SOLD