OS,
I have a 1980 C30 & being in the engineering design field, NEVER liked my orig. setup.
Here's why.
With the pump running, yea, the higher V-berth tank drained first into my setee tank & from there to my pump.
This also makes your pump work harder.
So, you now run from one tank thru the lower tank to your pump. This would be fine, but for an area where the hose leading back to the setee tank rises up higher at a point (for me in the hanging locker) than the setee tank inlet line. Here is where my hi-point was before running a bypass line alongside of the setee tank (inboard) & making the dual tie-in connection after the setee tank drain connection (the lower point) & knot draining from one tank into the next.
In this way, your pump's suction is more fluid & your tanks are draining.
With your current setup, you will only drain the v-berth tank to a certain level, as water between tanks will always seek its own equal level. Once the water in your V-berth tank falls below this, you will just suck AIR. So, you end up with a forward tank still 1/3+/- full yet, your setee tank is now empty & your pump is running dry.
On the suction side of a centrifugal pump, any run that loops higher than the pump suction line, can create an air lock. The pump is trying to suction fluid but cannot, as there is air trapped in the line leading to the pump.
Here is what I did. on my boat.
I re-ran the drain line from the V-berth tank so it had no vertical rise to trap flow. It took some drilling to cut down on any line-rise-looping. I routed this line alongside the inboard side of the setee tank. I then connected this line to the drain line from the setee tank (aft the tank). I tied them together with a "Y" fitting for connecting to the pump suction connection.
I don't have this sucking air problem now until both tanks are going on empty. There was not alot of quality control when they built my boat so, when I find these errors, I simply try & find an improvement.
Any fix may not be perfect but, I hope this gives you a better picture.
CR