Welcome to Justanswer.com, my name is Dustin.I'm sorry to hear about the issue you are having, I will do my best to help you resolve it.
The Rv 760 and Rv760 LMt are the same device.. Same software, same features. The only difference is one gets free lifetime map updates, and the other does not.
What you may need to do is connect your 760 to your computer and then update it, using this page
After you have connected the 760, and then downloaded and installed garmin express.. And then allowed garmin express to update the 760 unit.
You should then see the eco route option available in the apps.. However, you have to have the ecoroute HD mechanic unit in order to use it. Or you have to have the garmin mechanic app installed on your iphone or android phone.. And you have to pair the 760 to the device or app.
If you get stuck or run into any issues, let me know.
I strive to provide great service, and greatly appreciate a 5 star rating clicked and submitted when you are satisfied.. I always answer follow up questions.Dustin S.Senior Technician and Network Switching Systems Operator / Maintainer.