You can prevent problems that may be caused by residual liquid by diluting it with a sufficient quantity of nontoxic antifreeze. Never use permanent engine-type antifreeze in either of these systems; it will be harmful, even deadly, to you and to the environment. Remember, the less liquid that remains in these tanks, the less antifreeze you'll have to add. It'll be easier to flush the potable-water system in the spring, too. While it's not harmful, most folks find that the smell, taste, and foam caused by nontoxic antifreeze are unpleasant at best.
While it's important to run this antifreeze throughout both systems-through every pump, hose, drain, and faucet-it's best to avoid the water heater by installing a bypass loop. This involves nothing more than disconnecting from the tank the potable inlet and outlet hoses (but don't disconnect the engine heat-exchanger connections), then connecting them together with a length of hose or rigid plumbing. Then simply drain all of the water from the water heater; most units are equipped with a drain. In the spring, reverse the process.
The reason for this approach is twofold. Once nontoxic antifreeze enters many water heaters, it becomes difficult, if not impossible, to remove the odor and taste from the hot-water side of the system. Additionally, even small water heaters will ingest several gallons of antifreeze before it flows into the rest of the plumbing system.