Do we know for certain that it's only TP?
Not wet wipes or a tampon? 'Cuz just about anything else will dissolve if you can put some muriatic (hydrochloric) acid down the toilet for it to soak in overnight, and it won't harm any of the plumbing or the porcelain. Even an overload of quick dissolve TP will dissolve on it's own in just plain water in about an hour.
But I agree with Rich...you need to find out how much if any sea water mineral buildup has reduced the diameter of your head discharge line. The good news is, muriatic acid will also dissolve mineral buildup, and do so a whole lot faster than vinegar. A cupful of undiluted distilled white vinegar flushed ALL the way through the line to the tank once a week when the boat will sit will prevent it. Use only distilled white vinegar...not cider.