While Undertow does not provide this information for Trailer Life via TLDCN software here's some good news and some bad news....
First the good news. Check out this link!http://www.lowclearances.com/
Now the bad news. Trailer Life Directory Campground Navigator is not included in the provided list, but maybe your GPS is included and the GPS can be used along with your TLDCN system.
This site claims to have the most extensive database on low clearance points in the U.S.A. Maybe someone can convince Undertow or Trailer Life to include this in a future release. But please understand there are business arrangements, contracts, pricing and other obligations that may not get resolved to include this. As the site implies, this data can change all the time as new roads are built, overpasses created, construction impacts and this does NOT remove the concern of low clearance accidents, so one must still use some research and common sense. So no air surfing on top of your rig.... thwack!