Untested route below - the wildcard one have to be absolutely last portion of Url. So to force ".html" at the end you need to use constraint (5th argument).
routes.MapPageRoute(
"ManyParam",
"{*path}",
"~/Default.aspx",
false,
new RouteValueDictionary(),
new RouteValueDictionary { { "path", @".*\.html" } }
);