If you are going to carry any significant amount of water, it needs to be down low and as far forward as feasible.
I think washing hair every 4 days is a long way from excessive. The issue is probably the amount of water needed for rinsing. When my wife had long hair and we tried to conserve water, she put her head down in the sink and I used the sink sprayer on low to rinse her hair.
My other thought would be, how long is 30 gal going to last anyway? With navy type showers and serious water conservation, my wife and I go through about 5 gal per day for drinking, washing, washing dishes, etc. That is 6 days maximum and in warm Summer temps we are likely to be more like 5 days. Worst case, we can skimp on the showers, use paper plates, and I have a couple of 5 gal jugs. Maybe we could push past 10 days but that would be difficult.