wal mart has a product that lubes the valve. i clean my tanks and close them up and dump some of the conditioner in the empty tank. has worked for me. also do a good clean out when you dump. i always dump 5 gal of water down the toilet after i dump to get the desidue out. and i always see toilet paper in the out flow. that may be your problem closing down on residue.
2007 chevrolet 2500 duramax 6.6
allison 6 speed
2004 classic 28 WB , Pro Pride P-3 hitch
centramatics wheel ballancers 300-556