Holding Tank higher than toilet
Problems with doing this:
1. Pressure is greater than the pump is designed for to get water uphill. The packing around the piston wears out quickly and leaks, getting your hand wet as you pump with sewage and/or outside water.
2. Sewage will sit in the hose unless you pump a great deal of water through the system to clear the sewage, at least 20 strokes. Stinky smells develop.
We have this situation in our forward head. We like the fact that it empties by gravity when out in the ocean. But our Wilcox-Critendon head isn't strong enough to handle the situation. Maybe an electric toilet would be the answer. Holding tank would fill up pretty fast, however. We're still struggling with the odors, continuing to replace hosing, but the above problems remain.