Is a 40' RV too long for much of MX?

You just have to know where you are going. If you have a toad, use it to check a route our through small unfamiliar towns first to avoid gettign trapped in tight streets or dead ends. Look for overhead hazards, like signs or balconies sticking out to far, trees & low wires.

And don't try going down into Santa Maria del Oro crater with one.

One good trick is to follow buses through a town