On my 365, when i open the caps to the drains for front or rear black/grey tanks, i hear a vacuum seal like air tight pressure being released, then looks like some black spills out....just a little. When i put the cap on the tank valves are closed so i do not know why i get this "leak". When i open the cap the valves are closed as well. Anyone else have this issue? Any corrective measures i can take?
2012 Ford F-350 6.7L CC SRW w/FX4
2013 Forest River Sierra 365SAQ