Mine would "WEEP" and cause the pump to cycle every 2-3 minuets. This was at the rear toilet. The valve seeped and was hard to see. When on city water I never knew it. I would verify the tank level on city water and that might be easier by a long flush, leaving the drain valve(s) open for an extended period, then closing and rechecking again in a day or two. Good luck....
Don & Dona W/yorkies Gizmo, Master Chief, & Tali
2011 Flagstaff 8528 BHSS 2015 Ford 6.7 XLT
2010 Full Timers & Still Going