It is a septic holding tank…
if you want it to smell like lemons use it to make lemonade…
otherwise its going to stink either like septic or that awful smelling perfume made to put in it…
Best if the system has no problems and is designed right, if so, it will vent the odors to the outside and they will disperse into the outside air basically unnoticed…
Odors inside the trailer are a sign of a failure of some type, either in design, construction, or use…
Other than that water is all that is needed to keep it working efficiently and properly…