Originally Posted by BuddyRay
As Artie Johnson used to say on Laugh-in (dating myself here) "Veeerrrryyyy interestink".
I just looked at the specs and you only have one gray tank.
I'm guessing here - but I wonder if the drain pipe from your sink goes into the tank at a different spot (level) than the shower? That would be unusual as I think most TTs are plummed with everything joining together then entering the tank.
I guess its possible that you're getting some kind of vacuum lock. This can happen in pluming systems (of all types not just RV) when there just isn't enough "drop" between the bottom of the trap under the sink and the top of the water level in the holding tank. Basically what happens is that they isn't enough water in the line between the trap and the tank to "pull" the water through the trap. This can be made worse if the trap is partially full of stuff (like grease).
You could verify this theory by using an old fashioned (clean) toilet plunger on the sink the next time it backs up. If the plunger forces the water on through then it's not a full tank.
I guess there is also a chance that your level monitor isn't wired right - but that wouldn't explain why it's not backing up in the shower.
Your first paragraph was I thought I had as well, but you're thinking that everything transitions into the grey tank through one inlet...hmm
The trap is a S trap on the smaller side and then 90 degree elbow with a 1.5 foot horizontal segment of pipe pointed towards you as u stand in front of the sink (slightly off to the left as u stand in front of sink)..the sink is a part of slide out. I assume after that it's a drop down to the grey tank. This drop has to be at least 2.5 feet down through floor and into the grey tank. I think this drop would be enough no matter the depth of material in the grey tank. But, there can't be a hardline in the plumbing at some point behind the sink paneling to accomdate the operation of the slide out or how do slide-out kitchens have their plumbing set up?