Going on first rally with no hot water ?????

snowdonia

well after 10 years caravanning, have decided to take the plunge and try a rally, end of Aug bank holiday weekend at Holmfirth with the cheshire rally centre, £34 for a long weekend,  2 x adults , 1 x child 10 yrs, + 1 x puppy known as the beast ! .(bargain)..
have had a test run in the van at the weekend with hubby using battery only, all ok except NO hot water, wondering what we are doing wrong ? how do you get your hot water to work when using a battery ?.