Fixed a Valley recently that was missing one of the bolts that holds the aluminum cross members up. It was sagging in the middle even more than normal.
Wood cross members are going way back. Aluminum is more structurally sound and was an upgrade to the problems with wood. Most guy's don't check the tightness of the bolts because they can't see them.
My valley it's a 72 model with wooden crossmembers but it does not sag as bad as a aluminum crossmember that recently recover, so switching to aluminum did not fix the problem either?