It's normal for water to stay in the water heater, yes, although with some setups it can drain with the standard low point drains for the RV water system. Normally those wouldn't be opened when draining tanks.
There's no great reason I can think of to drain it when storing the RV during the camping season. It also saves some on the aggravation and (slight) expense of replacing my Atwood water heater drain plug quite so frequently. (Nor, for that matter, do I see any great need to drain the fresh water tank for relatively short-term storage.) Obviously the entire water system including the heater should be drained out for winterization.