Sink drain block when using holding tank (Thor 19g class c)

[Jmunroe08]

Kitchen sink drain worked fine yesterday when hooked up to wastewater drain, but today when only using gray tank, no drainage. Any ideas?

[CHS]

Check (Anti-Siphon) valve to see if it is blocked. It may not be letting air into the system thus not letting liquids to drain.
It is used in place of a standard vent to the roof type. When you use water it is supposed to open to let air into the system. If it's blocked it can keep water from draining. Happened to me a couple times.

[fhenn]

drain Grey water tank

[16ACE27]

Well since it always drains to the grey water tank, regardless of whether you are connected to a sewer connection, then the sewer connection has no bearing on the problem.

Does the shower drain? If not, Frank is probably correct.
If it does, then you either have a clog or CHS may have hit the nail on the head.

[Jmunroe08]

Thanks everyone. I realized we were not level, and pulled out the levelling blocks - after that, the sink drained and seems to be ok.
We'll see!
Multiple big learning curves for newbies, eh?!