An RV Heater is a smaller version of home water heater. It provides hot water in kitchen and bathroom.
For most RV renters this is not a critical part of their RV trip. as they can live with colf water or may have access to RV parks showers that have hot water supply.
However if you plan to stay at a cold place and need hot water then after your booking you should verify with the RV owner that the water heater works ok.
Since RV water heater take anywhere from 30-60 minutes to warm up, therefore you will not be able to verify it at the time of RV pick-up. However as soon as you rent the RV turn the switch on and check after an hour if water is warm enough for you.
As stated earlier if you are a new renter and are staying at a decent RV campground then you do not need to worry about RV water heater.