The PO was overly cautious...you can flush TP, provided you don't use too much (which, unless your wife and any other women in your family are very unusual women, will require retraining 'em 'cuz women are apparently genetically programed to use half a roll/flush)--only about 5-6 sheets at a time...AND, it's the right kind of TP. You don't have to spend the money for TP labelled marine RV, though..."Marine/RV" toilet paper is just private label branding of very cheap flimsy institutional grade toilet paper at a much higher price than you can buy it without any “marine/RV†labeling.
To find out whether any toilet paper is suitable for use aboard, tear off a sheet and put it in a glass of water overnight. If it's still an intact—or mostly intact--sheet in the morning, it's NOT suitable for use in any marine toilet...it doesn't dissolve fast enough and could clog your system. But if you stir the glass and all you see is "snow" and cloudy water, it's the same thing as "marine" toilet paper...and just fine for use in any system.
But even the "right" kind of toilet paper can clog a marine toilet if too much is used at once, so boat owners have to learn to use it as sparingly as possible. In situations where a little isn't enough, it may be necessary to use a little--flush that...and use a little more, flush again--even several times (and by the way, that doesn't ONLY apply to toilet paper).
However, NOTHING else should ever go down a marine toilet that hasn't been eaten first--not facial tissue, not even a toothpick...DEFINITELY not baby wipes or feminine sanitary products, waste and TP only. The label may say "flushable" or "septic tank safe," but that only applies to household toilets...marine toilets will choke on ANY thing other than waste and the right kind of TP in limited amounts.
Do not use any household chemical bowl cleaners in a marine toilet...nor any other household cleaning products that contain bleach, pine oil, or petroleum _base_d solvents...they're all highly destructive to the rubber parts in toilets and hoses.
A 20 gallon tank is fairly small--TOO small for your family of 5. A manual marine toilet averages at least 3/4 gal/flush. The average adult uses the toilet an average of 5 x/day (a lot of averaging here)...children may go more often. Just you and your wife alone will put 7-8 gallons/day into the tank...your 3 kids will put at least that much more into it. Do the math and it becomes immediately apparent that a 20 gal holding tank isn't big enough to see your family all the way through an entire weekend....you're gonna have to pump out every day to prevent overflowing the tank--which is a major no-no.
Whether the solution is larger tank or whether you can solve the problem by adding a Coast Guard certified Type I MSD (device that treats waste and discharges it overboard legally in all waters except those specifically designated "no discharge") depends on where the boat is--on a navigable river or a non-navigable "no discharge" lake.
There's a good bit more to operating and maintaining a marine toilet and holding tank that you need to know if you want to prevent problems instead of solving 'em. At the risk of being perceived as trying to sell you something--although it's actually my publisher who makes any money from it...I just get a small roylaty--I've written a book that covers just about everything you need to know. You can check it out by going to the _link_ in my signature. However, I'm always glad to answer questions whether you buy it or not...you just won't have as many if you do!
