Uh-oh...fixable, but you're not gonna like it...
First, you need access to the top of the tank. Then, if there isn't one, you'll need to retrofit an inspection port (kits available from SeaLand-- 800-321-9886 --for plastic tanks)...that will allow you access to the inside of the tank. It can be pumped out through the access port...THEN you can remove the clog from the discharge fitting. You MIGHT be able to blast enough water down the pumpout hose through the deck fitting to dislodge the paper towel and pump out the tank, but it's likely to be a "pump a little, blast it loose again, pump a little more, blast again" exercise...and you'll still have to get into the tank to get the paper towel out. There isn't anything that'll dissolve it in less than 6 months.Told ya you wouldn't like the answer.