fill all the water system as normal.plug in the EHU cable and test the water heater.doing it without water may cause damage to the heater at great expence.
------------- the only silly question is the one you do not ask.
I have done this it works on gas but can't seem to get it working on electric, proberly dosen't help that i have no instruction manuals, I have a thermostat with from left to right 1 gas symbol, 2 gas symbol, 2 lightening symbols up 1 lightening symbol down then a symbol like a tap. I have a primus gas boiler and the water heater is honeywell.
I think you will find that the Primus system is supported by Alde so it might be worth giving them a call and seeing if you can get a manual emailed to you. If in doubt get a caravan engineer who has experience of the Primus system rather than taking risks with gas and electric.
------------- 'A sure cure for sea-sickness is to sit under a tree'