[TAKPAK]

I have done what you are thinking about/suggesting on our 2011 3400. Same situation, "bathroom" grey is lower than the kitchen tank, but it works. I had to install the shutoff valve on the main outlet of the sewer line so the two tanks would "cross over". If we are somewhere without sewer connection, I close that main line valve, open both the bath and kitchen valves, and let the water build up in both. One thing, because the bath tank is higher than the kitchen, the bath tank will fill up first. It will stay that way (showing full) the whole rest of the time. Then what you need to watch is the level of the kitchen tank. When it gets full, you really ARE full. I'm not sure on your model, but on ours, if I forget to watch the level, because the shower and sink are higher than the kitchen sink, if you overflow it, you could have shower water coming up in the kitchen sink. Not as bad as black water, but not pleasant, either. So, you can do it, but watch your levels carefully. Also, NOT recommended to "backwash" the black tank with that water. Tried it once, was not pretty. Didn't blow anything, but the odor that came out of the vent on the roof was pretty bad when that water surged into the black tank. Plus, there is always the possibility of getting a backwash of black water into the grey tanks. So, not a good idea. Keep the black one separate. Doing what you want to do, we can go 4 days plus on the grey tanks. The black tank we can go 7 or 8 days easily. Hope this helps.