After considering all the options I went with the Road Shower 2.
A few weeks ago as I was looking up water tanks and shower options for the cruiser I came across the Road Shower 2. It is an extruded aluminum tank that stores water on the roof rack and then uses passive solar radiation to heat the water throughout the day. Its got a temp gauge, radiator cap for filling and to control pressure (18 psi max), an air valve to pressurize the tank and a hose/nozzle that has about 5' reach. It mounts with hardware using an integrated "T" slot on the bottom. Overall, its VERY well made and quite light for its size. It holds about 5 gallons of water, and my initial testing in the backyard yielded about 10 minutes of constant water flow with good pressure. It would obviously run out fast or last longer depending on the flow rate selected at the adjustable nozzle, but a HOT 10 minute shower isn't out of the question for this shower unit. I just tossed it up there for now but I will orient the hose to the rear so I can use it near the tailgate.
So far, in my humble testing, letting the tank sit in the backyard with direct sun on it for about 4 hours (Noon-4pm) with 85* ambient temps I was hitting right at 100* at the nozzle. Even at 9pm that night the water was still relatively warm (closer to 90*) and I started with cold tap water out of the faucet. Needless to say I am impressed and my wife is stoked about it! There's something about the idea of washing down after a day in Moab's silt, or Colorado's dusty trails that is really appealing.
All things considered, I only need to worry about the sun shinning (no biggie here in Colorado and Utah) and if its luke warm a pot of hot water from the stove should warm things up nicely. Another big advantage is it is out of the way on the roof rack and easy to access when desired. I am impressed. Give me a season off road with it next summer to know if it will stay or be replaced.
Full disclosure, I have no experience with the Zodi Showers, but I have used the solar shower bags in the past with varying results.