Likely a small leak at the connection, either at the hose, or inside where the plumbing connects. Are you certain it is coming from under the camper and not simply running down the outside? Also when you flush that way you are getting half the volume in each tank to do the cleaning work. Connect the hose individually and see which one may be the culprit.
Andy, Sherrie, Dixie, and Daisy
2020 Jayco 336FBOK Modern Farmhouse
2020 Chevy Silverado 2500 SRW Duramax 10 speed GM/ Allison
CURT Q20 on R20 Slider