Free online map that shows road/Interstate grades of slope?

I had an interesting experience in western Maryland and West Virginia. On the steep grades on the interstates, with just our TV, I needed to brake lest I hit 80 MPH. With the travel trailer, I needed no braking. The big sqareish front and back generate a bunch of drag and the TT isn't that heavy and I've read similar things from others.

Official interstate grades are not that big of a deal to a camper. Of course. A windy mountain road with 10 and 20 MPH switchbacks could challenge the brakes in some cases.