Hi avk84,
Thanks for posting your question here. Although we do not offer any tools that would allow you to apply your custom template to *all* 5000 of your saved listings at one time, you should be able to apply this template to up to 250 saved listings at a time using the "Find and Replace" option that is available on the Saved Listings page within your account.
From your Saved Listings page, just select "250" from the "Items/Page" drop down menu, check the "Select All" box to choose all the saved listings on the page, click the "Find and Replace" button, select "Listing Template" from the "Select Field to Edit" drop down menu and then proceed as directed.
You should be able to locate your custom template from the template selector within your account by selecting “Custom” from the list of categories in the upper left corner of the interface.
If you need any further assistance with this, please feel free to contact our Customer Support team by mousing over the “Help” tab and selecting the “File Support Case” option.
-Mike