In about the same size boat, I have twin tanks under the bed in the aft cabin and twin tanks in the lazarette. The bottom level of the laz tanks are above the top of the bed tanks. They are interconnected, so the top tanks are empty before the bed tanks start being used.
On a long trip, we go through the 150 gals of laz tanks in a week or so, then, the level of the bed tanks stars going down, and the bed tells us, by oil-canning when the weight of a person arrives on the mattress. Also, by then the boot stripe, at the transom, rises by a few inches.
Those clues are really all we need, to go for a fill up.
When filling, all of the deck fills are on the lazarette deck, so the bed tanks get filled first, then the laz tanks. We always fill to overflowing, so know we have as much as we can carry.
When buying this boat, the broker showed off the sight tubes and valves that would allow use of each tank separately, but we have never seen a reason to use any of that. Nor have I found any reason to remove the useless sight tubes.