X2 on a full septic system. Why pay to pump it every few months? If the trailer isn't used full time, you may never need to pump the system.
I stayed at an old Forest Service cabin that had a 55 gallon drum buried in the ground and a single perforated leach line leading out from the barrel. One of those big blue plastic barrels would work fine. Good septic tanks have a grease barrier at the top to keep grease from going into the leach line. Since grease floats, it would be easy to make one.
The EPA encourages evapotranspiration fields. The leach line is on top of a heavy plastic liner. We built some of those, and all kinds of things grow in that area.