Generally caravan handbooks are little more than 'Cut and Paste' content from the various appliance and equipment makers manuals, and as most of those are common across multiple brands and models of van, those manuals are usually far more easily obtained.
You will need to look at installed appliances, chassis and other equipment, to find the particular model number on the labelling, then do some Googling to find them. You will hopefully end up with about 90%, or more, of the content you would find in the caravan handbook.
Also, caravan handbooks tend to be rather generic across a range of models, and change little from year to year unless there was a major range/model update, so any handbook from the manufacturer roughly around the year of your van may well prove of use, and perhaps a little easier to find than a specific model and year.
I have the stuff for most of the equipment and chassis it is the water system is unusual on this as it has a tank and large water filling hole rather like a petrol tank.
Try asking a specific question about your water system 'problem', your's is not the only van with an onboard tank, likely someone can give some insight if not a complete answer.
I don't have an onboard tank, so probably not the best person to advise.
With most older caravans, mine is 1992, you often find that many modifications have been made, and your water system may well be one of them. Not necessarily though as some did have a built-in tank as an option. Obviously if yours has been modified in this way, no handbook will cover it. Water systems in caravans though are usually very simple in nature, so if you have a specific problem, just ask on here and I'm sure people will try and help.
I fitted a system like you describe to a caravan I had back in the 1980s. It had a tank under one of the berths along with an electric pump.
There are basically two methods used in caravans to provide water. One uses a pressurised system where the pump pressurises the water pipes when all the taps are closed and a pressure switch then turns the pump off. The pressure drops when any tap is opened and the pump starts up again. The other type uses microswitches in the taps and when any tap is opened the micoswitch turns the pump on. Both systems have their advantages and disadvantages. Whether the tank is built-in or external, either system can be used.
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