Septic tank safe doesn't necessarily mean
it will break down fast enough to keep from clogging up a holding tank discharge fitting. If you're in doubt about whether a toilet paper is ok to use aboard, tear off a sheet and put it in glass of water overnight. If it's still an intact--or mostly intact--sheet in the morning, keep looking. But if the water is milky and all you see is "snow" when you stir it, it's the same thing as high priced "marine" toilet paper. Two ply or single, same test applies.However, marine toilets cannot handle large wads of ANY toilet paper. It must be used sparingly, and occasionally "intermediate flushes" may be called for to keep from overloading the bowl with waste OR paper...or both.