I traveled all winter and stored my rig for 2 months while visiting in Houston. When I was getting ready to go I was draining the fresh tank on the pavement and I noticed little chunks of crud. At that point I dropped the panel under the fresh tank and could see a dark witness mark a few inches up in the tank. Well....I wasn't using it until I cleaned it so out comes the tank. It had a moldy looking black line around the inside. I used simple green and about 10 pounds of various nuts and bolts and used 2 people to tilt, roll and agitate and scrub until clean. I bought the rig 2 years ago and had the under covers off to see everything and it wasn't there at that time. I now bleach it and circulate/flush everything while sitting. I didn't worry too much about well water while using it all winter because we go through so much water. I would never start using a used rig until I'm 100 % sure the tank is clean. Yes, it was a pita but what are you gunna do?