I don't drive my truck much in the summertime. My battery died. Any ideas?

The GPS unit must be drawing some power even when it is off. Most modern vehicles will always have a very slight draw on the battery even when everything is "off". The same is probably true of you GPS unit. when you add that on top of the normal draw it could be causing your problem.

Get a solar charger ,will maintain your battery and they're reasonably priced ..