No, you never want to connect your sanitation system to any other system in any way. This is for health
reasons and smell reasons. Given the right conditions, fluids can flow the wrong way through a hose or pipe. The same applies to your gray water system, potable water system, engine cooling
system and your seawater washdown system. All should be isolated from one another so that there is no possibility that one can contaminate the other.
Also, never assume you can get away with it by using a check valve. Check valves can and do fail.
I agree with what Daddle says about the possibility of sucking air.