18 gal holding is a bit small for "no discharge"
inland lake...30 gallons is more appropriate for a boat that size. The trick will be finding space for a larger one that isn't so far away from the head that you'll always leave sewage in the hose to permeate it. However, 65 gallons of fresh water is more than adequate. I'm also on an inland "no discharge" lake...I've never carried more than 50 gallons water, and I've never run out, even on 3 day weekends with guests aboard AND a fresh water flush toilet, AND an icemaker. At home you have an infinite water supply...on a boat, there's a finite amount of water. You have to learn how to conserve it. If you just let water run as you do at home, 200 gallons won't be enough. Learn to turn it off when you're not actually using it. For instance, wet your toothbrush...turn off the water while you brush...turn it back on when you're ready to spit out the toothpaste and rinse your brush and the sink. If you shower aboard, you can't let the water run continuously...get wet, turn it off...lather up...turn it on to rinse off. Washing dishes: fill the sink or a dishpan with hot soapy water...turn the water off. Wash the dishes...turn the water on to rinse the suds off. Use ONLY the water you actually need, and you'll find that you have more than enough. Once you accept the idea that life on a boat requires doing a few things differently than you do on land, it just becomes second nature, and no hardship at all, to conserve water. That's when you'll be on your way to becoming a real sailor instead of just another d'd floating condo owner.