coffee grounds in the sink in an rv? y or n?

I don't do that. Coffee has a way of getting into everything, including your drain valve.

i wouldn't put grounds down any drain... RV or home.

Probably won't cause a problem once in awhile.

We added a rubber mat screen over our sink drain to catch any solid pieces. It works great and the solids are dumped in the trash.

Do not do that. Dump coffee grounds in the trash. Scrape plate into the trash before putting them in the sink. The less solids that go down an RV drain the better.

Not in the RV drain but they can be good for the garden.

We don't dump coffee grounds down any sinks.

NO solids at all into the drain. Everything drains into a tank. Solids typically settle to the bottom of the tank. It is much easier to remove solids from the basket strainer than to remove them from the bottom of the tank.

Nay, I can just see them getting stuck in the drain valve and keeping it from closing all the way.

You would need to treat the galley tank just like you do the black tank and NEVER dump it unless it's nearly full (in order to have a chance of flushing out the solids). While this practice MIGHT work, it still leaves the issue that the OPENING of the galley tank is much smaller than the black tank opening so on balance I'd say it's a bad idea.

Why would anyone dump the whole container of used coffee grounds in a sink? We use a paper filter in the coffee maker so disposal is as easy as opening the cabinet door and letting the whole mess fall in the trash can. I know some folks don't use a filter so washing the grounds container would be a tad more work but I don't understand throwing the whole thing in the sink.
Then again, it's job security for me, I will come to your house (or trailer) and repair the drain problem. If you want to see me soon, throw the whole macaroni casserole in the disposer and let it sit for a couple of days. I'll be right over!