cold showers is something i know a lot about,in our camping days we used a coleman cube portable hot water system, they work fine as they have an internal 12 volt battery(rechargable 12v or 240v) to run the pump and ignite the gas,they work fine, more so if you run the water through a couple of time to get the temp you want,just drop the shower head into the storage bucket until the temp you want is achieved. it actually uses less water than the caravan shower as you are not wasting the water in the van lines, the down side is the 1kg gas bottles are expensive over $10 retail,you can (probably not actually legal) refill them from a doner lpg cylinder or use a a larger cylinder if you purchase the correct connection hose (if memory serves me correct coleman fittings). the other downside is their size and having to find a space in van or tug, just for the odd trip, solar shower bags work ok ,if the weather is overcast you can add warm water from you kettle to achieve the desired temp, now you will have to look a portable ensuite tent so you dont scare the wildlife,