slow drain of freshwater tank

Recently purchased a class c and everything works fine except now I have a very slow drain when I try to drain the freshwater tank. I'm wondering if the last owner left water in the tank to long and their is a build up of algae in the tank and it has clogged the drain lines. I tried to run a wire thru the drain line but there are two elbows that the wire won't go passed. Is their something to put in the tank that will help clear up this problem?
Thanks for any suggestions you may have.