Having been the owner of a Minuette I feel I can help with a Moselle.
Of the pics, 19 is the fitting to connect a Whale submersible pump. The pump 12V is supplied through the two contacts near the bottom of the enclosure. I would suggest you clean them up a bit with either a fibreglass contact cleaning brush or a small piece of fine wire wool - if the latter make sure all 12V is switched off. Also clean the aperture thoroughly as the water will be used for washing. Th water container can make the water taste and you might get infections from it, so whilst it is OK for washing I would suggest you get a new second smaller container to carry drinking water.
The second pic - 20 - is indeed the inlet for the toilet cistern. There is no real need to put anything in the cistern but if you get flakes of debris when the toilet is flushed then a suitable (usually pink) additive in VERY small quantity is a good idea. Fill it with plain cold water up to the lip. However you should put additive in the toilet waste container to break down debris and to kill any smell. This liquid will be blue or green. Historically it used to contain Formaldehyde but that can destroy to operation of a septic tank if one is being used so always used one that does NOT contain that chemical. Green does not contain the chemical and most (but not all) blue ones don't contain it either.
I would expect the van to have main electrical hook-up (known as EHU). If it does you need to check it thoroughly (if you are suitable qualified or experienced) or get it done by an electrician (does not need to be a caravan specialist.) You can buy a suitable cable compliant with regs from any caravan dealer for maybe £20 or so. It will have blue 3-pin connectors (to CEE17 standard) on the ends and the cable will be 3-core flex of 2.5sqmm per core with a tough bright orange outer cover.
The caravan will likely have an under-counter fridge which can be run off mains or gas when you are on site, and from 12V when you are towing. The van will likely have two car connection plugs, one black and one grey. The black carries all of your road lighting, the grey carries 12V continuous for caravan battery function/charging, and switched 12V for the fridge which, since it draws nearly 7A, should only be powered when the towing vehicle engine is running. If there is only one 13-pin plug on the caravan that carries all connections, but if/when you have a towbar fitted you MUST specify to the installers (and ideally get it written and signed on the work order) that the socket needs fridge wiring which some systems omit.
Finally it is likely that your insurers will require you to have some sort of lock to stop theft. This can be a lock that fits on one wheel, and/or a device that clamps or fits around the towball mechanism. These can be expensive new, but you may be lucky and find a used one on line.
Good luck.
