I used to get the valves closed and drain every 2 or 3 days until a foot valve on the toilet stuck and overflowed into the coach messing up the control boards on the 2 furnaces.
The toilet overflowed or the valve was open and filled the black tank? Either way, unless kids were involved how was that missed by the "operator"?
I have since (2001) been leaving the valves open all the time.
Personally, take it for it is worth, you have caused your problem with this line of thinking. And you limit your options when you actually y do have a clogged or slow discharge.
When do the factories guys get involved?
There is no doubt the sewage engineers on this forum have way more experience and use of the sanitation systems then the blokes at Indy. And your best bets are going to be right here.
Every marine chemical product has a note about adding the product and then adding 5-10 gallons of water in the tank. By leaving that black valve open you will never get the benefit of the chems.
All right, time to get a "visual picture". The description has more to do with travel trailers because the bowl usually sits right on top of the black tank with a direct shot. 5th wheels, because the bowl is frequently on another level or some distance away usually involves some 3" plumbing to get to the tank.
But this thought is common to both. When you leave the black tank valve open all the time solids don't get to soften or break up with either chemical action alone or just the constant movement of the the water being flushed. So you get what we affectionately call a "s*it teepee". Get the picture? it will not drain properly and it is gross. Black tank flushing works best when the tank is nearly full. Obviously solids being heavier will be near the bottom with lots of water to help push things out.
Granted you don't have the same solids in a gray tank as you do with the black. But you do have some as well as a build of of soap scum, hair whatever.
By keeping your black and gray tanks open you can where it will be a problem. But by also keeping your gray tanks open you lose a potential plumbers aid with the black.
There have times I have noticed my black tank drain slow down sooner then I expected. Sometimes with the tank flusher running and closing the main drain for a minute or two allows enough additional water to help push out any blockages.
But I also use the buildup in gray tanks to help. Using the "flush king" valve mentioned above (highly recommend you get one), I can close off the that valve and with the black tank open then open my nearly full gray tank. This inrush of gray water rushing into the black tank helps move things around really good. Close the gray, let the black dump. I will usually do this twice, sometimes twice with both gray tanks.
OK, back to our regularly scheduled program - since you didn't ask about black tanks.
And as a side note, there have been a couple of instances where the hole was drilled in the tank for the drains and the piece was allowed to fall into the tank. That can cause some problems, but will usually still allow water to drain, albeit very slowly.
I forgot to mention that the sink and the shower in the front bathroom drain just fine, which probably means the blockage is between the sink and the holding tank.
Does your kitchen tank drain at all, even slowly? Try opening and closing the drain valve multiple time, that may help.
Are you sure that the valve is even moving when you pull the handle? Drop some screws holding the underbelly somewhere within a foot or two of where the drain valve is a and then follow the cable to the valve.
If you can get some water to drain, or you have a portable sump pump you could try filling both sink basins with water and then pull both sink drains, try and get some water movement.
I would be hesitant about using one of the "air or volume blaster" devices available at the the big box stores on an RV. And being all PVC with no where near the water flow of house hold systems I would be careful about using a snake in the pipes. Too much of a chance of damaging or scratching the pipes.
But, you could remove the roof vent over your sink area and possible see quite a ways down. Maybe use your snake there.
A less intrusive snake would one of the versions that you attach to a garden hose and use household water pressure. The I referring too has a bullet shaped end and several small jets that spin as water flows.
What ever snaking method, be aware that under your sink you have a vacuum breaker device that could fail and allow the slop to spill over under the cabinets. Get a flashlight and look up towards the back and high under the cabinet.
You could try a snake through the outlet pipe back to the tank, although you might to think that out before you try it.
Hope something here helps.
We went from my sink will not drain to where can we buy Calgon????? The 4 drain valves are open.
If any of the ideas above are helpful - then you need to go find the Calgon, do some through cleaning and maybe rethink your Sanitation device operations.