To clean out the tank, all you can do is drain it, add more water, drain it, add more water, drain it, add more water. Repeat, repeat, repeat until the tank is clean. Once it's cleaned out, run water through all the lines and flush, flush, flush. When no more foam in the water occurs, it's finally all cleaned out.
Last step, since it's been sitting so long, is to sanitize. (Bleach or city water as City Water has chlorine in the water.) The stronger the bleach, the faster it sanatizes. But the strong the bleach, the more caustic it is on valves, and such, So make sure you rinse well the bleach water after sanitizing too.
One more little tip. When you get ready to sanatize, fill the tank with water-bleach solution and take the camper out for a good drive. The sloshing in the tank will wash the top, sides, and bottoms and reach every speck inside the tank. Then drain after driving around. The beating around of the water will help break anything loose that might be clinging to the inside walls. So when filling, don't fill it all the way, leave some space (air) so it can slosh real good.
The driving and sloshing is probably the best way to pulverize and break up anything in both black and gray tank, and to clean your fresh tank really good.
