Hello sb6351 and welcome to the forum.
Yes, "holding" would be your black tank. The sensor in the tank is likely fouled. While it's an industry standard system, it's not that great. It's possible, but unlikely you'll get the sensor to read correctly.
Most of the time, RVers wait for the black (holding) tank to tell them when it's time to dump it. Often, there's a burp that happens when flushing the commode when it's time to empty. Other times, your nose may tell you.
For those of us who travel often, we dump more on a timed basis, the morning it's time to hit the road.
I see you're from Lubbock. That places you in our West Texas Chapter of the Heartland Owners Club. This chapter has a rally coming up in Abilene on March 16th. Consider joining the group there. Not a better way to learn more about your Heartland RV.
Here's a link to more information.
Consider also joining the Heartland Owners Club. It's fast and free!
Here's a link for more information.