Last year I replaced the entire system. Just yesterday in the boat enjoyed the "sweet" smell of rubber hoses, not leaking gases.
Replacement was prompted by a poorly placed holding tank, which included the vent line connected by silicone caulk ONLY (yes, a hole drilled in the tank and then the vent line caulked in place. I replaced it with Ronco Bow style tank, including building a floor in the bow to hold the tank. All new hoses and 1.5" vent lines. Raritan PHII head, with a new head floor thanks to newfound foam cored glassing skills. I enjoy having the v-berth filler area open again, and no smells.
Replacement was prompted by a poorly placed holding tank, which included the vent line connected by silicone caulk ONLY (yes, a hole drilled in the tank and then the vent line caulked in place. I replaced it with Ronco Bow style tank, including building a floor in the bow to hold the tank. All new hoses and 1.5" vent lines. Raritan PHII head, with a new head floor thanks to newfound foam cored glassing skills. I enjoy having the v-berth filler area open again, and no smells.