Looking for website that will create an elevation profile at, say 100m horizontal intervals, of a US driving route on google maps that's on the order of 1000 miles.
Try a cycling route planner like Ride With GPS
The USGS has an app that lets you overlay on a US map. It also let's you see an elevation profile along a multi-point line. But it does not let you select a specific roadway. https://viewer.nationalmap.gov/advanced-viewer/