Sounds like a definite problem with the flush valve. To narrow down possibilities - When you step on the foot pedal to flush, does the pedal quickly return to the fully up position? Have you tried pulling it fully up by hand or lifting it with your foot?
If the pedal doesn't come all the way up, the valve won't close and water will continue to flow. If the pedal does come all the way up and water still flows, the valve needs rebuilding or replacement.
Considering the age of your rig, it may also be something as simple as giving the pedal/valve assembly a thorough cleaning and a shot or two of lubricant.
Good luck, and welcome to the forum !
2019 Thor Freedom Elite 30FE
Living The Dream in "God's Waiting Room"