Good point about water flow based on gravity.
Our solar shower bags were fine though.. low flow helps conserve water.
If it doesn't work well enough getting a pump may be in order... with water saving shower head.
The issues we were running into that made me put together the tank were:
1. I can't transport water in solar bags... the bags would rip on the rough roads.
2. When I get to camp there is often no water, and if there is water, there is not enough solar time to heat it up before the sun sets.
So, I figured the tank would solve this:
1. I fill the tank when we encounter water. Typically creek water (drinking water is usually a premium and I like to keep it just for drinking, rather than also use it for a shower)
2. The water can just warm up during the day when we travel, including on rough roads.
3. This way we have water when we get to camp, and it is warm. This should solve my shower water problems.