24Laughs, Howdy;
It may be that the valve seals have dryed out. Can occour when they sit for a time with zero wet stuff and from not being used (exercised).
You
may need to replace them, or ... disassemble and lube with a silicone based lubricant.
Should you choose to re-place;
http://www.valterra.com/RV/rv-main.htm
hankaye
PS. My RV is same as yours, only 1 Model year older, front to back I have Gray (shower & lavy), Black, then the smaller 1 1/2" is the kitchen.
When I dump the tanks I usually toss 2 caps full of downey fabric softner down the bowl with 2 bowls of water. the fabric softner will "slick up" the valve's seal, and slick the tanks interior as it fills. Helps keep "things" moving and covers the foul oder nicely.
__________________
Striving for a less complicated life since 1949 ...
Home: 2008 Cougar 278 RKS
T.V.: 2004 F-250 4X4, Level III BulletProofed , Detroit Tru-Track Differential (915A550)
Dog: 2006 Border Collie (Rascal) aka Maximum fur dispersal unit. (08/04/2006 - 12/16/2017) RIP.