One simple way to do this is to purchase a simple plug-in timer (available at most stores like Wal-Mart & KMart) and a good quality extension cord. Plug the timer into your home or garage outlet, plug the extension cord into the timer and run this to your RV. You will need to use your adapter (or buy one) to plug your RV into the extension cord. If your power outlet is outdoors, you may want to plug in the extension cord first and then into the timer. This method allows you to keep the timer inside your power cord storage area in your RV where it will be out of the weather and then plug your RV into the timer. Either way works.

Set you timer to come one for an hour or so every day. When the timer comes on your on-board charger will keep the batteries fully charged and fully charged batteries are less susceptable to winter freeze damage.