You didn't say which model you have but it looks like replacing the battery is a snap. See this youtube video on
Replacing a Magellan 1440 Battery. It looks like
NewPower99.com is the place to Magellan batteries. It looks like all their batteries are $24 so this would be a pretty inexpensive thing to try
Another option is to connect the USB port to your computer and use the Magellan software to sync the saved locations. When you get a new unit I would imagine you can plug it into the computer and upload those waypoints to the unit.
On a side note I just realized Magellan has some really cool features. Their
Road mate Series for RV's has Good Sam parks pre-loaded along with ratings and other information. It looks like the Garmin products also have an RV park feature, but since I'm a Good Sam member the magellan looks very attractive.
KJ