What system would you recommend for hot water heating for a shower install in a VW Crafter MWB campervan? I have a diesel heater already for air heating. Kind regards

Ideally you should be considering a heater installation set similar the Webasto Thermo Top Evo with included plate heat exchanger see bultertechnik weblink https://www.butlertechnik.com/item/Webasto/Thermo-Top-Evo-5kW-Diesel-Camper-RV-Air-and-Water-12v-Heating-Kit/613

Of course if you already have an air heater installed for saloon heating you would not require the blower heater matrix in this kit unless you wanted to build into your install some redundancy / duality. It would be a shame maybe to install such a great heater just for an occasional shower