If the kitchen sink is in a slide, the flex hose can develop a kink in it that can restrict flow, especially in one slideout position. Did you notice if the plugged condition was worse when the slide was open or closed? A kink in that flex hose created a big problem for me when the crimped spot caused the hose to split in the crimp's corners and, unbeknownst to us, leak drain water under the tile flooring. Installers had cut the flex hose a few inches too short so that in the slide's out position the hose kinked at its lower pipe attachment rather than gently bend along its length, similar to a garden hose you fold back on itself; the longer the loop, the less likely the hose is to kink.
On ours, access to the flex hose assembly is via a removeable panel, on the aisle-side wall next to a pull-out "pantry" below the stove, that has hidden cabinet catches top and bottom, and that just pulls straight out by placing your fingertips behind its edges.
As to the vents, my coach doesn't have one for the kitchen or shower, but the two mid-coach bath sinks have the valves, and I think also the washer/dryer drain. They apparently are supposed to be enough, together with a tank roof vent, to prevent the traps from being sucked dry when you empty your gray tank. I'm not sure it works, since my ventless shower drain reeks after tank emptying and I have to add a couple cups of water to fill the shower trap to stop the odor. I should try to remember to put something over the shower drain before dumping tanks to limit trap evacuation I guess.
For whatever reason the kitchen sink trap doesn't seem to display that issue; perhaps it commonly gets used before tank fumes back up that pipe. Each trap in the coach should have been installed with an associated vent valve, I agree.
Joel