Hello Louise,
Hot water in a caravan used to be a luxury, but now it has become a standard feature.
I take Rays point, but if you do not fit it, them the experience may put you off caravanning, so the choice is yours.
With regard to the purchase of a second hand water heater, you must be very careful. One of the issues with caravans is the fact that for most of their life they are standing unused, often for months at a time. It is during these periods of inactivity when some parts of appliances start to corrode, and insects can find their way into the small crevasses, and can block gas burners etc. In the case of water heaters, they are very susceptible to frost damage, and the damage is not always immediately visible, and can be expensive to repair.
Unless you are fully familiar with these appliances (i.e. technically competent to service/repair) then it would be wise to only consider a second hand unit supplied with a proper guarantee by a dealer, or bite the bullet and purchase new or delay the purchase as suggested by Ray.
It is always the case that the fitting, removal, service or repair of any gas appliance must be carried out by a competent person. Incorrect fitting could endanger your life.