Paul,
I would check with Liberty before using a drain cleaner. It might damage the rubber seals on the sewer dump valve. I have never had much luck with drain cleaners anyway. Have you tried using a plunger? Sometimes that will push thru any blockage. If that fails, just undo the P trap under the sink and clean it out. If the slow drain still persists, maybe a plumber's snake run into the drain pipe after the P trap is removed might do the trick. The snake will clean the inside of the pipe of grease and sediment build up. Then you should be good to go!
Tuga & Karen Gaidry
2012 Honda Pilot