The best sensor for the black tank is eyeball it through the toilet.
it is hard to envision that you could ever fill the black tank in a week from those you can haul in your van.
When we are camping without sewer we often use the black tank by pouring dish water in the toilet to keep the gray from filling. Have gone 10 days this way
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles