The cylinder piston may be leaking past the seals or you have a valve leaking by. I would reinstall the battery and run the slides out and back in and see if that re-seats the valve if something had gotten in the valve keeping it from sealing tight.
I have a 2016 Sierra 5th wheel. Had the same problem plus the two front jacks started coming down while the unit was in tow. Camping World found that an actuator valve in the living room slide was leaking. A truck driver saw the front jacks coming down and alerted us. Be glad you weren't towing when you found your problem