You have to remember that once the Ultrastore is up to temperature, and that is 70 degrees if on electric, or a variable from 50 to 70 degrees depending what you have the thermostat set to, you have 10 litres of hot water.
As soon as you open the hot tap, the hot water is replaced by cold, which needs heating and takes time.
If your pump is rated at, for instance, 7 ltrs/min, then a couple of minutes and your 10 ltrs has gone.
You cannot run a van shower in the same way as a domestic home shower.