Hi S.W. Smith,
Welcome to the Heartland Owners Forum.
We have a 45 gallon black tank and it's usually getting full at 7 days (2 of us). If yours doesn't light up at all, you may have a problem with the sensor or test panel. Another possibility is that the dump valve has a slow leak (often caused by trapping waste material in the blade channel).
Next time you dump the black tank, close the dump valve and run the black tank flush for 5 minutes. Unless you have very low water flow, that should put 10-20 gallons in the tank. Use the timer on your phone and don't get distracted if someone walks up to talk with you. If the test panel still shows no lights, it'll need to be serviced.
Don't forget to let the water out of the tank.
The test panel lights are usually pretty useless as a little paper or waste gets stuck to the sensor and it always reads full. Most of us just dump the tanks on a schedule. Another method is to wait until the toilet "burps" while flushing. That indicates the tank is full.