Septic field pipes (NOT the tank, the fields) are typically at least 14-18 inches below grade. It is generally OK to run a light truck over them. Consider how they are built, first the field is excavated to grade, then the pipes are installed and covered with gravel, finally the soil cover is put on. This is all done using heavy equipment. The tank itself is usually reinforced concrete, and rated H-10 or H-20, depending on the manufacturer. An H-10 or H-20 rated tank can support a pickup truck easily. Of course, you may have an old tank that is not rated, so best to check your plans before driving on top of the tank.
Great info. The home was built in 1998. Do you think there would be anything old-school about the field, or would that age weaken it at all? I sense that I'd be fine towing this camper across that area...but just want to be sure. I won't be going anywhere near the tank.