The holding tank is made from the same sorts of plastics as are used in house (and RV) drain plumbing. Strong organic solvents (MEK and such like) would be the main thing to avoid, and you wouldn't normally have those in soaps anyhow.
Ideally you also don't want stuff that will coagulate or settle out and stick to the tank to avoid gumming things up too much. Again, that shouldn't be a big problem with body washes.
Sugar definitely wouldn't be a problem because it is completely soluble in water (and any shower is going to have more than enough water use to dissolve whatever amount of sugar would be in the body wash). I can't imagine the sugar having any useful cleansing action, but that's another story. Maybe it makes the soap taste a bit better for kids who use foul language.