Because portable portapotties have sealed (unvented) tanks, the most popular holding tank products won't work in 'em. Most of the Thetford products are lethal chemicals that don't do much more than add a chemical odor to the sewage odor. So you may have to try a few of the others to find one that you like. I suggest you start with Odorlos...its active ingredient is nitrates, which promotes oxygen release from organic matter--essential in PREVENTING odor--and is also environmentally environmentally friendly. It's available from a few marine retailers, but you'll find it for a LOT less in RV supply stores.
There's no reason to avoid putting solid waste in your portapotty.... contrary to popular belief, urine is NOT sterile and can be just as stinky as solid waste. Besides...solid waste is 75% water and dissolves in water very quickly.
Nor is it necessary to avoid flushing toilet paper...it just needs to be "quick dissolve" TP. That doesn't mean you have to use over-priced "marine/rv" TP though...almost any of the cheapest no-name stuff will work. There's a simple test to find out if it's ok for use in a marine toilet OR a portpotty: tear off a couple of sheets and put 'em in a glass of water. Come back in an hour and stir the water. If the water is "milky" and all you see is "snow," it's fine for use in any marine toilet/potty...but if the sheets are still intact or mostly intact, try another brand.
--Peggie