bathroon sink keeps plugging

Look at where your bathroom sink drains in to the black tank and I am sure that you will find your blockage. It should be right under the toilet drop in to the black tank. Many bathroom sinks drain in to the black tank to help flush away the solid waste that drops down from your toilet. This sometimes creates a pyramid of poop. This waste build up can clog the drain from the bathroom sink, especially if allowed to harden after a trip.

norsk129 wrote:
Thanks for the advice mine does not have the vacuum actuator.
from the p-trap there's only 6 inches of pipe before it enters the vent pipe. So it "shouldn't" need one right?

Take off your P-trap and put a bucket under your sink. Use the drain stopper to plug the sink while you put some water in it (less water than the bucket will hold, obviously). Open the drain plug and see if your drain is still slow.

If so, the issue is your drain stopper apparatus. It probably has a couple of "lift" settings and it might not be opening far enough to drain as fast as you'd like. Or the apparatus itself may be gunky.

If that isn't the issue, keep looking for an AAV. You likely have one in the line somewhere. I've not seen many bathroom sinks without one.

Seems like you've checked everything else (roof vent & p-trap) except whether or not the sunk plumbing line or the camper itself is so off level it can't drain well.

I've done that already too. The pipe is clean :-(

I have to ask, have you removed the P-trap and looked inside?
When you have the p-trap removed you can run your snake down into the gray tank. that should do it. (If you have a wife with long hair you will be surprised what you find in the trap.) Been there, done that.
Tell us what you find.

Thanks for the advice mine does not have the vacuum actuator.
from the p-trap there's only 6 inches of pipe before it enters the vent pipe. So it "shouldn't" need one right?

You can get them at Lowes or Home Depot.

Look under your sink for your air admittance valve (AAV). When they manlfunction they can cause a slow drain and/or unpleasant smells.

Here's a link to a post from TusconJim that shows what a couple AAV's look like. Most campers come with the all-black (less expensive) type shown on the right.

Ranger Smith wrote:
Vacuum breaker under sink is stuck closed

+1

Does it drain right in to the black tank near the toilet drop? Some bathroom sinks do and that may be where your problem is.

Vacuum breaker under sink is stuck closed