The changes that actually returned meaningful weight for me, in order of what they gave back per unit of effort:
Smaller water tank. I went from a large tank to about half and gained the most of any single change. In practice I was never running it dry between fills anyway, I was carrying a reserve I never used, everywhere, all the time. Work out what you actually consume between fills over a week rather than what feels safe.
Thinner ply and less framing. 12mm ply on walls and ceiling can very often be 6mm or 9mm with sensible battening. Retro-fitting this is a rebuild, so it is a lesson for the next one rather than a fix for this one.
Leaving the second water tank empty unless heading somewhere remote. Free, immediate, and reversible.
Being honest about the gear. I was carrying a full tool kit, a spare of several things, and books. Two boxes came out and stayed out.
Lithium instead of lead if you have not already.
The habit worth building: weigh at each stage of the next build, not at the end. A bathroom scale under each item before it goes in, written on a list. You cannot manage this at the end; the whole problem is that it accumulates invisibly.