Here is a link of someone with a similar problem. https://www.irv2.com/forums/f107/tec...sh-465304.html
Have you inspected the tank sensor for the toilet? Sounds like either a controller problem, or a sensor problem, or a dirty tank problem. https://www.thetford.com/wp-content/...enceRV-Eng.pdf
In the interim, if you are sure the black tank isn't near full, you can follow page 15 unlock feature, which will disable the lockout process.