Hello Sean,
David has given some sound advice, but there are some alternatives, and some conditions you must be aware of:.
Lets deal with the heater. As David said the Carver C2 has not been manufactured for about 8 years now. Truma Purchased the Leisure products division of Carver and then closed it down after about 6 months. Some spare parts are still manufactured, but they will be becoming more scarce as time goes by. Gary on this forum has a wealth of knowledge on Carver products and the availability of spares.
That leaves two main contenders for the actual heater. The Truma Ultrastore or the Propex Malaga. Both require careful fitting, and becuase they are gas applinces the fitter must be competent to fit gas appliances working to the current gas regulations. so unless you are Gas competent legally it is not a DIY job.
As for the water side, The heater takes a feed from the cold water supply already in the caravan. You will need to provide separate hot water piping from the heater to the sinks & basin. It is your choice if you go for a mixer or separate taps but they must seal properly.
You only need 1 pump. The simplest is to use a submersible pump that drops into your water container to lift the cold water. The cold supply enters the caravan and it them meets a pressure switch (Whale is a common supplier for this) After the pressure switch the cold water pipe divides. One leg goes to sink cold taps,and the other feeds the cold inlet of the water heater.
The pressure switch has two functions. it contains a non return valve that prevent water from running back to the water barrel, and it locks off the supply pipework in the caravan. This enables the pump to deliver water when the taps are open, but when the taps are closed the pump continues for a while as it pressurises the caravan pipework. the second function is the actual pressure switch.
When the pressure is low, the switch contacts close. If the pump is wired through the switch That will cause the pump to run. When the taps are turned off; the the pump will continue to run until the pressure is enough to open the switch contacts which stops the pump.
There is an alternative inboard pump. These are called diagram pumps (rather like your foot pump) these usually come with both a pressure switch and none return valve built into them. They tend to be bigger and nosier, but it could use your existing cold water inlet pipe and replace your foot pump. They can usually self prime over a reasonable distance because they use positive displacement technology.
With either of these systems if you run out of cold water the pump will continue to run as they cannot pressurise air. With the submersible pump this can happen and you may not know because the pumps are relatively quiet. The submersible pump will over heat and be damaged it it runs without water for more than about a minute. The diaphragm pump is usually more durable,
With these system you can have as many taps on the system as you like.
All the storage water heaters (Carver, Propex Truma) utilise the principal of displacement to deliver the hot water from their tanks. This works by first filing the tank with cold water. The maximum level the water reaches in the tank is determined by the level of the outlet which is near the top of the tank. There has to be some air gap left at the top tank to allow space for the water to expand as it heats up.
The cold inlet also has a non-return valve to independantly prevent the tank contents from emptying back into the cold water supply. So once the heater is filled it remains filled until you take deliberate action to drain it through a special valve.
To get hot water out you have to have pump cold water in. So even if you run out of cold water, you can rest assured that the hot tank will still be full although you can't actually get any water our until you restore the cold supply.
It is good practice to have an indicator to show when the pump is powered and should be running (12V led in parallel with the pumps power lead) You should also have a separate isolating power switch for the pump useful at night or when leaving the caravan for any length of time.
From time to time a pressurised system may operate for few seconds even though no one is using a tap. This can be either the hot water in the heater cooling down and contracting which is quite normal, but if the pump regularly trips in over an extended time, that probably indicates a leak somwhere in the system.
I hope this helps
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