How do I track the source of a site conversion if a user leaves the original landing page (that has the UTM tracking code on)?

I want to be able to track the source of a site conversion if a user leaves the original landing page that has the UTM tracking code on.

Example:
1. User clicks my google ad that leads to www.mysite.com/free-trial?utm_campaign=xyz where the end goal of this page is form submission for a free trial.

2. User lands on www.mysite.com/free-trial but wants to navigate around the site to www.mysite.com/products, leaving the UTM tracking behind (this shows up as a 'drop off' on google analytics, as the user isn't followed through the site, which is what I want).

3. User goes from www.mysite.com/products to www.mysite.com/pricing then finally back to www.mysite.com/free-trial.

4. By the time the user is back on the free trial page (www.mysite.com/free-trial), I want the UTM tracking ?utm_campaign=xyz to still be there for traceability.

Is there a piece of code that I can implement or something that I can do in Google Tag Manager/Analytics to be able to track the users moves through the site so I can trace where they came from and what steps they took before converting?

I have had a look into software like Hotjar for Heatmaps and visitor recordings, which I may have to use but would rather a more robust, easier to use solution! 

Any help/advice welcome, thanks!