Hello Trevor,
All the parts you will need are readily available through your local caravan dealership or in many cases through the internet.
The vast majority of touring caravans use a storage hot water heater. This usually contains about 9L (2 gallons) of water, and is usually dual fuelled by either LPG or a low wattage electric element. These are not instantaneous, so you need to have the heater turned on about half to three quarters of an hour before you shower.
Your current coldwater store is most likely an external barrel. This should be fine and will act as you cold supply. The system will also use just one pump, and again your existing pump may be suitable, if not it just needs replacing with a more powerful model.
The plumbing is quite straight forward, but if you intend to use a gas powered water storage heater then that must be installed by a competent LPG gas fitter.