It’s all about the simplicity of a diesel heater you push a button and you have hot water,no running cables in to a van for hours 5 minutes for me not 5 odd hours.
......... and you are only heating the water you are using. However, on the downside, we are also using diesel that is generating heat, +- 50% of which is going out of the exhaust. There have been attempts to capture that waste heat, but I've yet to see a viable solution.
The downside with electric is that you heat the tank of water in advance overnight without knowing if you can work the following day due to bad weather, etc.