I think you will find that most in line heaters are rated at 2000 w or less. I have one that is rated at 1400 w. It wont cope with cold water through a twin jet floor wand.
If you get a connector to go in the end of you hose and put an old fairly big jet in it, when you fill with cold water put the jet to return to the fresh tank and put the pump and heater on. This will warm the tank while prepping sufficiently to give hot water when you want it. It takes about 10-15 mins.
Trevor