The maps installed in my Montana 700i (City navigator that came with the unit) used to show on Basecamp. They no longer do.
How can I get them back to display in Basecamp?
Try reinstalling BaseCamp.
Usually this is due to a corrupt map cache, reinstalling BaseCamp won't fix that.
Clean up the map cache with JaVaWa Device Manager: https://www.javawa.nl/jdm_en.html